OpenJDK 1.8.0_191
I compiled and decompiled a piece of code below using Fernflower.
public class Decompile {
public static void main(String[] args) {
final int VAL = 20;
System.out.println(VAL);
}
}
The output is:
public class Decompile {
public static void main(String[] args) {
boolean VAL = true;
System.out.println(20);
}
}
I'm confused, how did VAL become a boolean?
UPDATE:
In Intellij IDEA decompiled code looks like this:
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
public class Decompile {
public Decompile() {
}
public static void main(String[] args) {
int VAL = true;
System.out.println(20);
}
}
The bytecode is
L0
LINENUMBER 5 L0
BIPUSH 20
ISTORE 1
L1
LINENUMBER 6 L1
GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
BIPUSH 20
INVOKEVIRTUAL java/io/PrintStream.println (I)V
As you can see the BIPUSH pushes 20 onto the stack, then ISTORE takes the value and store it into the local variable.
It's a Fernflower problem.
For your interest the output for bytecode version 55 is
int VAL = true;
System.out.println(20);
You can see decompilers can be wrong :)
The underlying issue is that Java bytecode has no notion of booleans, byte, chars, or shorts (except in type signatures). All local variables with those types are instead compiled to ints. Boolean true and false are compiled to 1 and 0 respectively.
What this means is that the decompiler has to guess whether a given local variable was supposed to be a boolean or an integer type. In this case, the value 20 is stored in the variable, which will never be stored in a variable of boolean type in Java code, so it should be easy for the decompiler to guess that it is an integer type based on the context. But it appears that Fernflower's boolean guesser is not that sophisticated.
For what it's worth, this is an inherently hard problem. Especially when you consider that non-Java bytecode doesn't have to follow the same patterns that Java does. It is perfectly valid for bytecode to use the same variable in both integer and boolean contexts. The Krakatau decompiler has a pretty sophisticated inference step for guessing whether variables should be booleans or not, but it will still get things wrong in situations like this.
It works like that as compiler do some optimization during the generation of the byte code. As VAL = 20; is final and not changing, so it can put the 20 in place of VAL without impacting the functionality in the second statement. Now the decompiler has only byte code and when it goes to read the Byte code it found 20 as inline in the second line. Byte code generated by the code as below:
0: bipush 20
2: istore_1
3: getstatic #20 // Field java/lang/System.out:Ljava/io/PrintStream;
6: bipush 20
8: invokevirtual #26 // Method java/io/PrintStream.println:(I)V
Related
I'm developing a code with lots of iterations
and I was wondering which one of these conditions is more efficient.
//1
Boolean.FALSE.equals(x)
//2
x == false
//3
!x
I am using the first one but i am not sure about it. If someone can give some information and help me I will appreciate it.
The second and third one shoud be the fastest. The first one involves extra overhead, although it could well be that the JIT compiler optimises it.
The issue is more about readability. The first one is practically unreadable.
At popular request, I've expanded the answer a bit. I wrote this class:
package com.severityone.test;
public class Main {
public static void main(String[] args) {
final boolean x = false;
final boolean a = Boolean.FALSE.equals(x);
final boolean b = x == false;
final boolean c = !x;
}
}
This is the resulting byte code:
Compiled from "Main.java"
public class com.severityone.test.Main {
public com.severityone.test.Main();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: getstatic #2 // Field java/lang/Boolean.FALSE:Ljava/lang/Boolean;
5: iconst_0
6: invokestatic #3 // Method java/lang/Boolean.valueOf:(Z)Ljava/lang/Boolean;
9: invokevirtual #4 // Method java/lang/Boolean.equals:(Ljava/lang/Object;)Z
12: istore_2
13: iconst_1
14: istore_3
15: iconst_1
16: istore 4
18: return
}
What we can see from here is that numbers 2 and 3 take two instructions each, whereas number 1 takes five. For most programs, it won't make any difference, but if you're running in a tight loop, it could make a difference.
As for readability, the adage of "less is more" goes. Because my eyes aren't exactly 100%, I have problems with large amounts of text, so I prefer to put plenty of whitespace in my code. If you need to write Boolean.FALSE.equals(x), and it's actually the x that you're interested in, you need to mentally swap the whole thing around.
As for the other two, readability is mostly a matter of personal preference. There's something to be said for all three options:
!x
x == false
false == x
The ! can be easy to overlook if you have a very long expression, such as !((value < 0 || value > 10) && "yes".equals(response)). Some people prefer to write ... == false or false == ..., because you don't easily miss it.
So, pop()method in java framework java.util.Stack class looks like this:
#SuppressWarnings("unchecked")
public synchronized E pop() {
if (elementCount == 0) {
throw new EmptyStackException();
}
final int index = --elementCount;
final E obj = (E) elementData[index];
elementData[index] = null;
modCount++;
return obj;
}
The part that I have trouble in understanding is local variable index. It seems we don't need it. elementCount is a instance variable in Vector class which Stack class extended.
So my point is,
final int index = --elementCount;
final E obj = (E) elementData[index];
elementData[index] = null;
These 3 lines of code can be written like
final E obj = (E) elementData[--elementCount];
elementData[elementCount] = null;
which consumes less memory, because memory space for index local variable isn't used.
Also, I found this pattern along the java framework source code. For example add(E Object) method in java.util.ArrayList class looks :
#Override public boolean add(E object) {
Object[] a = array;
int s = size;
if (s == a.length) {
Object[] newArray = new Object[s +
(s < (MIN_CAPACITY_INCREMENT / 2) ?
MIN_CAPACITY_INCREMENT : s >> 1)];
System.arraycopy(a, 0, newArray, 0, s);
array = a = newArray;
}
a[s] = object;
size = s + 1;
modCount++;
return true;
}
in this example, array is a instance variable, and as you can see, a new local variable a is assigned to hold it.
Does anybody know about this? Big Thanks in advance. :)
Though this is a really old question, but I want to share some information I earned during my journey.
I could find some explanation about my question on Performance Tips on Android page. First see sample code from the page,
static class Foo {
int mSplat;
}
Foo[] mArray = ...
public void zero() {
int sum = 0;
for (int i = 0; i < mArray.length; ++i) {
sum += mArray[i].mSplat;
}
}
public void one() {
int sum = 0;
Foo[] localArray = mArray;
int len = localArray.length;
for (int i = 0; i < len; ++i) {
sum += localArray[i].mSplat;
}
}
public void two() {
int sum = 0;
for (Foo a : mArray) {
sum += a.mSplat;
}
}
According to the above page, zero() is slowest, one() is faster. Because it pulls everything out into local variables, avoiding the lookups.
I think this explanation might solve my second question, which was asking "a new local variable a is assigned to hold it. but why?"
I hope this might help someone who have the same curiosity.
[EDIT] Let me add some details about "lookups".
So if you compile above code and disassembles the class file with javap command with -c option, it will print out disassembled code, i.e., the instructions that comprise the Java bytecodes.
public void zero();
Code:
0: iconst_0 // Push int constant 0
1: istore_1 // Store into local variable 1 (sum=0)
2: iconst_0 // Push int constant 0
3: istore_2 // Store into local variable 2 (i=0)
4: goto 22 // First time through don't increment
7: iload_1
8: aload_0
9: getfield #14 // Field mArray:[LTest$Foo;
12: iload_2
13: aaload
14: getfield #39 // Field Test$Foo.mSplat:I
17: iadd
18: istore_1
19: iinc 2, 1
22: iload_2 // Push value of local variable 2 (i)
23: aload_0 // Push local variable 0 (this)
24: getfield #14 // Field mArray:[LTest$Foo;
27: arraylength // Get length of array
28: if_icmplt 7 // Compare and loop if less than (i < mArray.length)
31: return
public void one();
Code:
0: iconst_0 // Push int constant 0
1: istore_1 // Store into local variable 1 (sum=0)
2: aload_0 // Push this
3: getfield #14 // Field mArray:[LTest$Foo;
6: astore_2 // Store reference into local variable (localArray)
7: aload_2 // Load reference from local variable
8: arraylength // Get length of array
9: istore_3 // Store into local variable 3 (len = mArray.length)
10: iconst_0 // Push int constant 0
11: istore 4 // Store into local variable 4 (i=0)
13: goto 29 // First time through don't increment
16: iload_1
17: aload_2
18: iload 4
20: aaload
21: getfield #39 // Field Test$Foo.mSplat:I
24: iadd
25: istore_1
26: iinc 4, 1
29: iload 4 // Load i from local variable
31: iload_3 // Load len from local variable
32: if_icmplt 16 // // Compare and loop if less than (i < len)
35: return
These instructions are a bit unfamiliar, so I looked up in JVM spec documents. (If you are curious, especially chapter 3, Compiling for the Java Virtual Machine, and chapter 6, The Java Virtual Machine Instruction Set would be helpful).
I added comment to help you understand, but in a nut shell, method zero() should operate getfield instruction on every iteration. According to JVM spec documentation 3.8. Working with Class Instances section, getfield operation performs several jobs like below.
The compiler generates symbolic references to the fields of an
instance, which are stored in the run-time constant pool. Those
run-time constant pool items are resolved at run-time to determine the
location of the field within the referenced object.
These 3 lines of code can be written like
We're in the business of making a useful and extendable programs, and in order to acheive that we should make our life as Developers easy as we can.
If it takes me a 5 more seconds to read the code and i can simplify it, i would. Specially if it comes in the expense of a int memory.. hardly calls as Optimization.
in this example, array is a instance variable, and as you can see, a new local variable a is assigned to hold it. Does anybody know about this?
This is hardly calls as question, i believe you meant to phrase it like that:
Why does they used another reference to array called a if they could use array ?
Well, I truly can't see why, because they could have use the E type since it given to them. It may be a reason of Covariance and Contravariance but i'm not sure.
Tip: Also next time you add pieces of a language source code, it will be nice to know which JDK you are viewing and a link me very help.
Keep in mind that --elementCount does the assignment before decrement. That means the fragment:
final int index = --elementCount;
final E obj = (E) elementData[index];
elementData[index] = null;
Can be translated into
final int index = elementCount;
elementCount--;
final E obj = (E) elementData[index];
elementData[index] = null;
Which means in your proposed replacement "elementData[--elementCount]" and "elementData[elementCount]" do not reference the same item. Your proposed replacement is not equivalent.
Hope this helps.
So, lets say I have this piece of code:
int mode = android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.HONEYCOMB ? AudioManager.MODE_IN_COMMUNICATION : AudioManager.MODE_IN_CALL;
Now, lets say I run this code on some device that is pre-gingerbread.
Is there any case in which the non-available static import of AudioManager.MODE_IN_COMMUNICATION would be hit?
What I mean is, is there any scenario in which I would see a crash due to the MODE_IN_COMMUNICATION which is not available pre gingerbread being checked?
How does the ternary operator compile in Java? Does it compile these two things as ints? Does it expand the code during compilation?
A static final "variable" that is known at compile time is compiled into your code under certain circumstances. (e.g. every int where the compiler knows the final value)
So your code is actually just
int mode = android.os.Build.VERSION.SDK_INT >= 11 ? 3 : 2;
And any version can run that. It's not making any references to the constants that may or may not exist on the Android device.
The technical details can be found within the Java Language Specifiction e.g. §13.1
References to fields that are constant variables (§4.12.4) are resolved at compile time to the constant value that is denoted. No reference to such a field should be present in the code in a binary file
You can see from the documentation if something is such a constant value.
Build.VERSION_CODES.HONEYCOMB "Constant Value: 11"
AudioManager.MODE_IN_COMMUNICATION "Constant Value: 3"
AudioManager.MODE_IN_CALL "Constant Value: 2"
Build.VERSION.SDK_INT is itself a static final int but is not inlined at compile time. The documentation does not state a constant value
Build.VERSION.SDK_INT
It is implemented as
public static final int SDK_INT = SystemProperties.getInt("ro.build.version.sdk", 0);
and the compile can't figure out what SystemProperties.getInt will return so this value is the only one that actually references a value from within your device.
Scroll to the bottom of my answer to see what the actual javac source code does :)
The Bytecode Generated
#zapl's answer definitely answers the specific question, but I feel like the OP's question still wasn't answered. How exactly does JVM compile ternary operators? So, I just want to answer that question for everyone wondering.
We can figure this out by looking at the actual bytecode generated. So, I created a test where I have two outside classes that have some static variable I'm referencing, and all that, but that's still besides the point because we just want to know if it compiles it the same way as an if-else. Regardless, I did a test with ternary and with the equivalent if-else and these are the results.
Java Code:
class main {
public static void main(String[] args) {
int a = 0;
int b = 2;
int c = a > b ? MyBigClass.VAR_1 : MyOtherBigClass.VAR_2;
//int c;
// if (a > b) {
// c = MyBigClass.VAR_1;
// } else {
// c = MyOtherBigClass.VAR_2;
// }
}
}
class MyBigClass {
public static int VAR_1 = 0;
}
class MyOtherBigClass {
public static int VAR_2 = 1;
}
As you can see I commented out the if-else for the test with the ternary, and then I just commented out the ternary when I was testing the if-else. The bytecode that resulted was this.
Bytecode using if-else:
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iconst_2
3: istore_2
4: iload_1
5: iload_2
6: if_icmple 16
9: getstatic #2 // Field MyBigClass.VAR_1:I
12: istore_3
13: goto 20
16: getstatic #3 // Field MyOtherBigClass.VAR_2:I
19: istore_3
20: return
Bytecode using the ternary:
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iconst_2
3: istore_2
4: iload_1
5: iload_2
6: if_icmple 15
9: getstatic #2 // Field MyBigClass.VAR_1:I
12: goto 18
15: getstatic #3 // Field MyOtherBigClass.VAR_2:I
18: istore_3
19: return
The resulting bytecode literally has just one extra instruction, which is storing the result in the first branch of the if-statement (wherease the ternary just stores the result at the end of the comparison). So, ternaries only execute the branch that would be followed according to the evaluation of the argument, just like an if-else statement.
And, because I was curious, I decided to check if a double ternary is equivalent to an if-elif-else. (Spoiler it is). I used this code to test it, same process as above:
public static void main(String[] args) {
int a = 0;
int b = 2;
int c = 3;
int d = a > b ? MyBigClass.VAR_1 : a > c ? MyOtherBigClass.VAR_2 : 0;
// int d;
// if (a > b) {
// d = MyBigClass.VAR_1;
// } else if (a > c) {
// d = MyOtherBigClass.VAR_2;
// } else {
// d = 0;
// }
}
The bytecode generated for if-elif-else:
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iconst_2
3: istore_2
4: iconst_3
5: istore_3
6: iload_1
7: iload_2
8: if_icmple 19
11: getstatic #2 // Field MyBigClass.VAR_1:I
14: istore 4
16: goto 35
19: iload_1
20: iload_3
21: if_icmple 32
24: getstatic #3 // Field MyOtherBigClass.VAR_2:I
27: istore 4
29: goto 35
32: iconst_0
33: istore 4
35: return
The bytecode generated for ternary:
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iconst_2
3: istore_2
4: iconst_3
5: istore_3
6: iload_1
7: iload_2
8: if_icmple 17
11: getstatic #2 // Field MyBigClass.VAR_1:I
14: goto 29
17: iload_1
18: iload_3
19: if_icmple 28
22: getstatic #3 // Field MyOtherBigClass.VAR_2:I
25: goto 29
28: iconst_0
29: istore 4
31: return
What Javac Source Code Actually Does
For the bold the brave and the few
I decided to look at the source for javac... It took awhile, but with a little help from their hitchhiker's guide to javac, I was able to find the one line that definitively determines what happens. Check it out (Line 914): https://hg.openjdk.java.net/jdk9/jdk9/langtools/file/65bfdabaab9c/src/jdk.compiler/share/classes/com/sun/tools/javac/parser/JavacParser.java
Do you see that? Let me clarify this a bit, lines 905-918 say this:
/** Expression1Rest = ["?" Expression ":" Expression1]
*/
JCExpression term1Rest(JCExpression t) {
if (token.kind == QUES) {
int pos = token.pos;
nextToken();
JCExpression t1 = term();
accept(COLON);
JCExpression t2 = term1();
return F.at(pos).Conditional(t, t1, t2);
} else {
return t;
}
}
The comment tells us this is what they use for parsing ternary expressions, and if we look at what it returns, it returns a conditional where t is the expression being evaluated, t1 is the first branch, and t2 is the second branch. Let's take a look at Conditional just to be sure. It looks like Conditional is being called from F, which if we dig a little deeper we can find out is the TreeMaker, what is a tree maker you may ask? Well, it's specifically an Abstract Syntax Tree which is often used as an intermediate representation of the code being parsed (check it out here https://en.wikipedia.org/wiki/Abstract_syntax_tree). Anyways, if we look inside that file (https://hg.openjdk.java.net/jdk9/jdk9/langtools/file/65bfdabaab9c/src/jdk.compiler/share/classes/com/sun/tools/javac/tree/TreeMaker.java) we can see at lines 306-313 this:
public JCConditional Conditional(JCExpression cond,
JCExpression thenpart,
JCExpression elsepart)
{
JCConditional tree = new JCConditional(cond, thenpart, elsepart);
tree.pos = pos;
return tree;
}
Which further confirms exactly what we thought, that a ternary expression is compiled exactly the same as an if-else statement (otherwise known as a conditional statement) :) I encourage anyone interested to take a look at the hitchhiker's guide (https://openjdk.java.net/groups/compiler/doc/hhgtjavac/index.html) and the code, it's actually really interesting to see how even a commercial grade compiler follows a lot of the principle things that you learn about in your standard compiler course at college.
The Java compiler literally replaces it with an if else block. I remember reading about this in a book during Programming Fundamentals I or II.
return isValid ? foo : bar;
literally precompiles to
if(isValid) {
return foo;
} else {
return bar;
}
which is then compiled as normal.
I was making some tests with numeric conversions and casts in Java and I found this strange behaviour (for me)
class Test {
public static void main(String[] args) {
//int y = 100000000000000; //does not compile
int x = 100000 * 1000000000; //compile (why?)
System.out.println(x); //x is 276447232
}
}
Basically x and y should be the same number: why does it compile?
Integer overflow goes undetected in Java, that's why the multiplication works fine. However, the literal you were stating was too large and thus is a compiler error.
I guess while the vast majority of Java compilers will precompute that value at compilation time that is not required by the JLS. Thus it cannot be an error, since then different compilers may compile a piece of code or yield errors – something that isn't so nice.
The Sun Java compiler actually performs the calculation as the disassembly shows:
Compiled from "x.java"
class x extends java.lang.Object{
x();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: ldc #2; //int 276447232
2: istore_1
3: getstatic #3; //Field java/lang/System.out:Ljava/io/PrintStream;
6: iload_1
7: invokevirtual #4; //Method java/io/PrintStream.println:(I)V
10: return
The important thing to note here is that for all intents and purposes the result must be the same as if it were computed during runtime. Therefore no compiler error can occur here.
Java deals with overflowing an integer not by an exception but by instead flipping over to negative values and continuing upward.
As an example, here's this code snippet:
int x = Integer.MAX_VALUE + 1;
System.out.println(x);
Java deals with it the best it can. That snippet outputs:
-2147483648
Which is the value of Integer.MIN_VALUE. You just have to be careful when you are dealing with really big numbers. That's why Java has a BigInteger class to deal with arbitrarily big numbers that go beyond the limit of integers.
This isn't meant to be subjective, I am looking for reasons based on resource utilisation, compiler performance, GC performance etc. rather than elegance. Oh, and the position of brackets doesn't count, so no stylistic comments please.
Take the following loop;
Integer total = new Integer(0);
Integer i;
for (String str : string_list)
{
i = Integer.parse(str);
total += i;
}
versus...
Integer total = 0;
for (String str : string_list)
{
Integer i = Integer.parse(str);
total += i;
}
In the first one i is function scoped whereas in the second it is scoped in the loop. I have always thought (believed) that the first one would be more efficient because it just references an existing variable already allocated on the stack, whereas the second one would be pushing and popping i each iteration of the loop.
There are quite a lot of other cases where I tend to scope variables more broadly than perhaps necessary so I thought I would ask here to clear up a gap in my knowledge. Also notice that assignment of the variable on initialisation either involving the new operator or not. Do any of these sorts of semi-stylistic semi-optimisations make any difference at all?
The second one is what I would prefer. There is no functional difference other than the scoping.
Setting the same variable in each iteration makes no difference because Integer is an immutable class. Now, if you were modifying an object instead of creating a new one each time, then there would be a difference.
And as a side note, in this code you should be using int and Integer.parseInt() rather than Integer and Integer.parse(). You're introducing quite a bit of unnecessary boxing and unboxing.
Edit: It's been a while since I mucked around in bytecode, so I thought I'd get my hands dirty again.
Here's the test class I compiled:
class ScopeTest {
public void outside(String[] args) {
Integer total = 0;
Integer i;
for (String str : args)
{
i = Integer.valueOf(str);
total += i;
}
}
public void inside(String[] args) {
Integer total = 0;
for (String str : args)
{
Integer i = Integer.valueOf(str);
total += i;
}
}
}
Bytecode output (retrieved with javap -c ScopeTest after compiling):
Compiled from "ScopeTest.java"
class ScopeTest extends java.lang.Object{
ScopeTest();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public void outside(java.lang.String[]);
Code:
0: iconst_0
1: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
4: astore_2
5: aload_1
6: astore 4
8: aload 4
10: arraylength
11: istore 5
13: iconst_0
14: istore 6
16: iload 6
18: iload 5
20: if_icmpge 55
23: aload 4
25: iload 6
27: aaload
28: astore 7
30: aload 7
32: invokestatic #3; //Method java/lang/Integer.valueOf:(Ljava/lang/String;)Ljava/lang/Integer;
35: astore_3
36: aload_2
37: invokevirtual #4; //Method java/lang/Integer.intValue:()I
40: aload_3
41: invokevirtual #4; //Method java/lang/Integer.intValue:()I
44: iadd
45: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
48: astore_2
49: iinc 6, 1
52: goto 16
55: return
public void inside(java.lang.String[]);
Code:
0: iconst_0
1: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
4: astore_2
5: aload_1
6: astore_3
7: aload_3
8: arraylength
9: istore 4
11: iconst_0
12: istore 5
14: iload 5
16: iload 4
18: if_icmpge 54
21: aload_3
22: iload 5
24: aaload
25: astore 6
27: aload 6
29: invokestatic #3; //Method java/lang/Integer.valueOf:(Ljava/lang/String;)Ljava/lang/Integer;
32: astore 7
34: aload_2
35: invokevirtual #4; //Method java/lang/Integer.intValue:()I
38: aload 7
40: invokevirtual #4; //Method java/lang/Integer.intValue:()I
43: iadd
44: invokestatic #2; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
47: astore_2
48: iinc 5, 1
51: goto 14
54: return
}
Contrary to my expectations, there was one difference between the two: in outside(), the variable i still took up a register even though it was omitted from the actual code (note that all the iload and istore instructions point one register higher).
The JIT compiler should make short work of this difference, but still you can see that limiting scope is a good practice.
(And with regards to my earlier side note, you can see that to add two Integer objects, Java must unbox both with intValue, add them, and then create a new Integer with valueOf. Don't do this unless absolutely necessary, because it's senseless and slower.)
The second one is far better because the first style is should only be used in C code as its mandatory. Java allows for inline declarations to minimize the scope of variables and you should take advantage of that. But you code can be further improved:
int total = 0;
for (String str: stringList) {
try {
total += Integer.valueOf(str);
} catch(NumberFormationException nfe) {
// more code to deal with the error
}
}
That follows the Java code style convention. Read the full guide here:
http://java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html
It makes no significant difference apart from on the last iteration, when the reference is cleared quicker in the second example (and that would be my preference - not so much for that reason, but clarity.)
Keep the scope to the minimum possible. The hotspot VM does escape analysis to determine when references are no longer accessible, and on the basis of this allocates some objects on the stack rather than on the heap. Keeping scope as small as possible aids this process.
I would ask why you're using Integer instead of a simple int...or perhaps it's just by way of example?
The second is far better. Scoping variables as narrowly as possible makes the code far easier to read and maintain, which are much more important overall than the performance differences between these examples, which are trivial and easily optimized away.
Neither. Integer.valueOf(0); will use a reference to a cached 0. :)
Well, in this case, you're instantiating an Integer primitive every single time you say i = Integer.parseInt(str) (where i is an Integer), so (unless Java knows how to optimize it), both cases are almost equally inefficient. Consider using int instead:
int total = 0;
for (String str : string_list)
{
int i = Integer.parseInt(str);
total += i;
}
Now we're back to the question of whether to put the int declaration on the inside or outside. Assuming the Java compiler has a lick of decent optimization, I'd say it doesn't matter. Efficiency aside, it is considered good practice to declare variables as close as possible to their use.
The second one is the preferable of the two for readability, maintainability, and efficiency.
All three of these goals are achieved because you are succinctly explaining what you are doing and how your variables are being used. You are explaining this clearly to both developers and the compiler. When the variable i is defined in the for block everyone knows that it is safe to ignore it outside of the block and that the value is only valid for this iteration of the block. This will lead the to the Garbage Collector being able to be able to easily mark this memory to be freed.
I would suggest not using Integer for intermediate values. Accumulate the total as an int and after the loop create the Object or depend on auto-boxing.
Assuming you have positive numbers in your list and you're serious with
I am looking for reasons based on
resource utilisation, compiler
performance, GC performance etc.
rather than elegance.
You should implement it by yourself like:
import java.util.ArrayList;
import java.util.List;
public class Int {
public static void main(String[] args) {
List<String> list = new ArrayList<String>();
list.add("10");
list.add("20");
int total = 0;
for (String str : list) {
int val = 0;
for (char c : str.toCharArray()) {
val = val * 10 + (int) c - 48;
}
total += val;
}
System.out.print(total);
}
}
The only GC relevant thing would be toCharArray() which could be replaced by another loop using charAt()
The question of what variable scope to use is a readability issue more than anything else. The code is better understood when every variable is restricted to the scope where it is actually used.
Now, if we inspect the technical consequences of using wide/narrow scopes, I believe that there IS a performance/footpring advantage with narrow scopes. Consider the following method, where we have 3 local variables, belonging to one global scope:
private static Random rnd = new Random();
public static void test() {
int x = rnd.nextInt();
System.out.println(x);
int y = rnd.nextInt();
System.out.println(y);
int z = rnd.nextInt();
System.out.println(z);
}
If you diassemble this code (using javap -c -verbose {class name} for example), you will see that the compiler reserves 3 slots for local variables in the stack frame structure of the test() method.
Now, suppose that we add some artificial scopes:
public static void test() {
{
int x = rnd.nextInt();
System.out.println(x);
}
{
int y = rnd.nextInt();
System.out.println(y);
}
{
int z = rnd.nextInt();
System.out.println(z);
}
}
If you diassemble the code now, you will notice that the compiler reserves only 1 slot for local variables. Since the scopes are completely independent, each time x,y or z are used, the same slot #0 is used.
What does it mean?
1) Narrow scopes save stack space
2) If we are dealing with object variables, it means that the objects may become unreachable faster, therefore are eligible for GC sooner than otherwise.
Again,note that these 2 "advantages" are really minor, and the readability concern should be by far the most important concern.
Second one since you want to keep the scope of your variables as "inner" as possible. The advantage of smaller scope is less chance for collision. In your example, there's only a few lines so the advantage might not be so obvious. But if it's larger, having the smaller-scope variables definitely is more beneficial. If someone else later has to look at the code, they would have to scan all the way back to right outside the method definition to know what i is. The argument is not much different than that of why we want to avoid global variable.