This question already has answers here:
What is an efficient way to implement a singleton pattern in Java? [closed]
(29 answers)
Closed 5 years ago.
When I learn guava,I found some code like this in com.google.common.hash.Hashing class:
public static HashFunction crc32c() {
return Crc32cHolder.CRC_32_C;
}
private static final class Crc32cHolder {
static final HashFunction CRC_32_C = new Crc32cHashFunction();
}
I want to know, why do not write like below, Is this merely the author's habit? or for other purpose?
public static HashFunction crc32c() {
return new Crc32cHashFunction()
}
Your alternative suggestion
public static HashFunction crc32c() {
return new Crc32cHashFunction()
}
would create a new Crc32cHashFunction instance each time crc32c() is called. If there is no specific need for a new instance to be returned by each call, it is more efficient to return the same instance in each call.
Using the static final variable
static final HashFunction CRC_32_C = new Crc32cHashFunction();
is one way to achieve a single instance.
As to why the HashFunction instance is a member of the nested class Crc32cHolder (as opposed to being a member of the outer class), the motivation is probably lazy evaluation - only at the first time the crc32c() method is called, the Crc32cHolder class would be initialized and the Crc32cHashFunction instance would be created. Thus, if that method is never called, the Crc32cHashFunction would never be created (assuming there is no other access to the Crc32cHolder class).
Related
This question already has answers here:
Why can I access my private variables of the "other" object directly, in my equals(Object o) method
(2 answers)
Closed 3 years ago.
I have two objects from the same class, calling it 'first' and 'second'. I have a method that takes in an object, so I use the 'first' object, call that method, pass in the 'second' object into that method.
Inside that method, why can I access the private instance variable of the 'second' object? Am I making any sense?
// Day.java, basic example of my question
public class Day{
private int stuff = 1;
public Day(int stuff){
this.stuff = stuff;
}
public int m(Day d){ // This method takes in an object as a parameter
int add = 0;
add = this.day + d.day; // why can you do this? isn't "day" private?
return add;
}
}
Source https://www.geeksforgeeks.org/access-modifiers-java/
You are accessing it from the same class
Check this Table Out..
Because you're still accessing your variable from within the same class it belongs to.
private variables are not visible from outside the class: but they are fully visible from within the class to which they belong.
If you tried to instantiate your class inside a different class, you'd have to use a getter to gain access.
Building a multi-language application in Java. Getting an error when inserting String value from R.string resource XML file:
public static final String TTT = (String) getText(R.string.TTT);
This is the error message:
Error: Cannot make a static reference to the non-static method getText(int) from the type
Context
How is this caused and how can I solve it?
Since getText() is non-static you cannot call it from a static method.
To understand why, you have to understand the difference between the two.
Instance (non-static) methods work on objects that are of a particular type (the class). These are created with the new like this:
SomeClass myObject = new SomeClass();
To call an instance method, you call it on the instance (myObject):
myObject.getText(...)
However a static method/field can be called only on the type directly, say like this:
The previous statement is not correct. One can also refer to static fields with an object reference like myObject.staticMethod() but this is discouraged because it does not make it clear that they are class variables.
... = SomeClass.final
And the two cannot work together as they operate on different data spaces (instance data and class data)
Let me try and explain. Consider this class (psuedocode):
class Test {
string somedata = "99";
string getText() { return somedata; }
static string TTT = "0";
}
Now I have the following use case:
Test item1 = new Test();
item1.somedata = "200";
Test item2 = new Test();
Test.TTT = "1";
What are the values?
Well
in item1 TTT = 1 and somedata = 200
in item2 TTT = 1 and somedata = 99
In other words, TTT is a datum that is shared by all the instances of the type. So it make no sense to say
class Test {
string somedata = "99";
string getText() { return somedata; }
static string TTT = getText(); // error there is is no somedata at this point
}
So the question is why is TTT static or why is getText() not static?
Remove the static and it should get past this error - but without understanding what your type does it's only a sticking plaster till the next error. What are the requirements of getText() that require it to be non-static?
There are some good answers already with explanations of why the mixture of the non-static Context method getText() can't be used with your static final String.
A good question to ask is: why do you want to do this? You are attempting to load a String from your strings resource, and populate its value into a public static field. I assume that this is so that some of your other classes can access it? If so, there is no need to do this. Instead pass a Context into your other classes and call context.getText(R.string.TTT) from within them.
public class NonActivity {
public static void doStuff(Context context) {
String TTT = context.getText(R.string.TTT);
...
}
}
And to call this from your Activity:
NonActivity.doStuff(this);
This will allow you to access your String resource without needing to use a public static field.
for others that find this in the search:
I often get this one when I accidentally call a function using the class name rather than the object name. This typically happens because i give them too similar names : P
ie:
MyClass myclass = new MyClass();
// then later
MyClass.someFunction();
This is obviously a static method. (good for somethings)
But what i really wanted to do (in most cases was)
myclass.someFunction();
It's such a silly mistake, but every couple of months, i waste about 30 mins messing with vars in the "MyClass" definitions to work out what im doing wrong when really, its just a typo.
Funny note: stack overflow highlights the syntax to make the mistake really obvious here.
You can either make your variable non static
public final String TTT = (String) getText(R.string.TTT);
or make the "getText" method static (if at all possible)
getText is a member of the your Activity so it must be called when "this" exists. Your static variable is initialized when your class is loaded before your Activity is created.
Since you want the variable to be initialized from a Resource string then it cannot be static. If you want it to be static you can initialize it with the String value.
You can not make reference to static variable from non-static method.
To understand this , you need to understand the difference between static and non-static.
Static variables are class variables , they belong to class with their only one instance , created at the first only.
Non-static variables are initialized every time you create an object of the class.
Now coming to your question, when you use new() operator we will create copy of every non-static filed for every object, but it is not the case for static fields. That's why it gives compile time error if you are referencing a static variable from non-static method.
This question is not new and existing answers give some good theoretical background. I just want to add a more pragmatic answer.
getText is a method of the Context abstract class and in order to call it, one needs an instance of its subclass (Activity, Service, Application or other). The problem is, that the public static final variables are initialized before any instance of Context is created.
There are several ways to solve this:
Make the variable a member variable (field) of the Activity or other subclass of Context by removing the static modifier and placing it within the class body;
Keep it static and delay the initialization to a later point (e.g. in the onCreate method);
Make it a local variable in the place of actual usage.
Yes u can make call on non-static method into static method because we need to remember first' we can create an object that's class we can call easyly on non -static method into static mathod
Building a multi-language application in Java. Getting an error when inserting String value from R.string resource XML file:
public static final String TTT = (String) getText(R.string.TTT);
This is the error message:
Error: Cannot make a static reference to the non-static method getText(int) from the type
Context
How is this caused and how can I solve it?
Since getText() is non-static you cannot call it from a static method.
To understand why, you have to understand the difference between the two.
Instance (non-static) methods work on objects that are of a particular type (the class). These are created with the new like this:
SomeClass myObject = new SomeClass();
To call an instance method, you call it on the instance (myObject):
myObject.getText(...)
However a static method/field can be called only on the type directly, say like this:
The previous statement is not correct. One can also refer to static fields with an object reference like myObject.staticMethod() but this is discouraged because it does not make it clear that they are class variables.
... = SomeClass.final
And the two cannot work together as they operate on different data spaces (instance data and class data)
Let me try and explain. Consider this class (psuedocode):
class Test {
string somedata = "99";
string getText() { return somedata; }
static string TTT = "0";
}
Now I have the following use case:
Test item1 = new Test();
item1.somedata = "200";
Test item2 = new Test();
Test.TTT = "1";
What are the values?
Well
in item1 TTT = 1 and somedata = 200
in item2 TTT = 1 and somedata = 99
In other words, TTT is a datum that is shared by all the instances of the type. So it make no sense to say
class Test {
string somedata = "99";
string getText() { return somedata; }
static string TTT = getText(); // error there is is no somedata at this point
}
So the question is why is TTT static or why is getText() not static?
Remove the static and it should get past this error - but without understanding what your type does it's only a sticking plaster till the next error. What are the requirements of getText() that require it to be non-static?
There are some good answers already with explanations of why the mixture of the non-static Context method getText() can't be used with your static final String.
A good question to ask is: why do you want to do this? You are attempting to load a String from your strings resource, and populate its value into a public static field. I assume that this is so that some of your other classes can access it? If so, there is no need to do this. Instead pass a Context into your other classes and call context.getText(R.string.TTT) from within them.
public class NonActivity {
public static void doStuff(Context context) {
String TTT = context.getText(R.string.TTT);
...
}
}
And to call this from your Activity:
NonActivity.doStuff(this);
This will allow you to access your String resource without needing to use a public static field.
for others that find this in the search:
I often get this one when I accidentally call a function using the class name rather than the object name. This typically happens because i give them too similar names : P
ie:
MyClass myclass = new MyClass();
// then later
MyClass.someFunction();
This is obviously a static method. (good for somethings)
But what i really wanted to do (in most cases was)
myclass.someFunction();
It's such a silly mistake, but every couple of months, i waste about 30 mins messing with vars in the "MyClass" definitions to work out what im doing wrong when really, its just a typo.
Funny note: stack overflow highlights the syntax to make the mistake really obvious here.
You can either make your variable non static
public final String TTT = (String) getText(R.string.TTT);
or make the "getText" method static (if at all possible)
getText is a member of the your Activity so it must be called when "this" exists. Your static variable is initialized when your class is loaded before your Activity is created.
Since you want the variable to be initialized from a Resource string then it cannot be static. If you want it to be static you can initialize it with the String value.
You can not make reference to static variable from non-static method.
To understand this , you need to understand the difference between static and non-static.
Static variables are class variables , they belong to class with their only one instance , created at the first only.
Non-static variables are initialized every time you create an object of the class.
Now coming to your question, when you use new() operator we will create copy of every non-static filed for every object, but it is not the case for static fields. That's why it gives compile time error if you are referencing a static variable from non-static method.
This question is not new and existing answers give some good theoretical background. I just want to add a more pragmatic answer.
getText is a method of the Context abstract class and in order to call it, one needs an instance of its subclass (Activity, Service, Application or other). The problem is, that the public static final variables are initialized before any instance of Context is created.
There are several ways to solve this:
Make the variable a member variable (field) of the Activity or other subclass of Context by removing the static modifier and placing it within the class body;
Keep it static and delay the initialization to a later point (e.g. in the onCreate method);
Make it a local variable in the place of actual usage.
Yes u can make call on non-static method into static method because we need to remember first' we can create an object that's class we can call easyly on non -static method into static mathod
Building a multi-language application in Java. Getting an error when inserting String value from R.string resource XML file:
public static final String TTT = (String) getText(R.string.TTT);
This is the error message:
Error: Cannot make a static reference to the non-static method getText(int) from the type
Context
How is this caused and how can I solve it?
Since getText() is non-static you cannot call it from a static method.
To understand why, you have to understand the difference between the two.
Instance (non-static) methods work on objects that are of a particular type (the class). These are created with the new like this:
SomeClass myObject = new SomeClass();
To call an instance method, you call it on the instance (myObject):
myObject.getText(...)
However a static method/field can be called only on the type directly, say like this:
The previous statement is not correct. One can also refer to static fields with an object reference like myObject.staticMethod() but this is discouraged because it does not make it clear that they are class variables.
... = SomeClass.final
And the two cannot work together as they operate on different data spaces (instance data and class data)
Let me try and explain. Consider this class (psuedocode):
class Test {
string somedata = "99";
string getText() { return somedata; }
static string TTT = "0";
}
Now I have the following use case:
Test item1 = new Test();
item1.somedata = "200";
Test item2 = new Test();
Test.TTT = "1";
What are the values?
Well
in item1 TTT = 1 and somedata = 200
in item2 TTT = 1 and somedata = 99
In other words, TTT is a datum that is shared by all the instances of the type. So it make no sense to say
class Test {
string somedata = "99";
string getText() { return somedata; }
static string TTT = getText(); // error there is is no somedata at this point
}
So the question is why is TTT static or why is getText() not static?
Remove the static and it should get past this error - but without understanding what your type does it's only a sticking plaster till the next error. What are the requirements of getText() that require it to be non-static?
There are some good answers already with explanations of why the mixture of the non-static Context method getText() can't be used with your static final String.
A good question to ask is: why do you want to do this? You are attempting to load a String from your strings resource, and populate its value into a public static field. I assume that this is so that some of your other classes can access it? If so, there is no need to do this. Instead pass a Context into your other classes and call context.getText(R.string.TTT) from within them.
public class NonActivity {
public static void doStuff(Context context) {
String TTT = context.getText(R.string.TTT);
...
}
}
And to call this from your Activity:
NonActivity.doStuff(this);
This will allow you to access your String resource without needing to use a public static field.
for others that find this in the search:
I often get this one when I accidentally call a function using the class name rather than the object name. This typically happens because i give them too similar names : P
ie:
MyClass myclass = new MyClass();
// then later
MyClass.someFunction();
This is obviously a static method. (good for somethings)
But what i really wanted to do (in most cases was)
myclass.someFunction();
It's such a silly mistake, but every couple of months, i waste about 30 mins messing with vars in the "MyClass" definitions to work out what im doing wrong when really, its just a typo.
Funny note: stack overflow highlights the syntax to make the mistake really obvious here.
You can either make your variable non static
public final String TTT = (String) getText(R.string.TTT);
or make the "getText" method static (if at all possible)
getText is a member of the your Activity so it must be called when "this" exists. Your static variable is initialized when your class is loaded before your Activity is created.
Since you want the variable to be initialized from a Resource string then it cannot be static. If you want it to be static you can initialize it with the String value.
You can not make reference to static variable from non-static method.
To understand this , you need to understand the difference between static and non-static.
Static variables are class variables , they belong to class with their only one instance , created at the first only.
Non-static variables are initialized every time you create an object of the class.
Now coming to your question, when you use new() operator we will create copy of every non-static filed for every object, but it is not the case for static fields. That's why it gives compile time error if you are referencing a static variable from non-static method.
This question is not new and existing answers give some good theoretical background. I just want to add a more pragmatic answer.
getText is a method of the Context abstract class and in order to call it, one needs an instance of its subclass (Activity, Service, Application or other). The problem is, that the public static final variables are initialized before any instance of Context is created.
There are several ways to solve this:
Make the variable a member variable (field) of the Activity or other subclass of Context by removing the static modifier and placing it within the class body;
Keep it static and delay the initialization to a later point (e.g. in the onCreate method);
Make it a local variable in the place of actual usage.
Yes u can make call on non-static method into static method because we need to remember first' we can create an object that's class we can call easyly on non -static method into static mathod
This question already has answers here:
What's the best way to share data between activities?
(14 answers)
Closed 9 years ago.
I have a json file. So, I was parsed the json file and I stored all value in an ArrayList. Now, I want to share the value all the classes in application. So, what is recommended and efficient way to do that??
Singleton pattern.
just create a public static reference for your List object and access it from any part of your app. it will be accessible as long as your app is alive. and you can deallocate it manually (by nulling the reference).
public class SharedData {
public static List<Custom> jsonData;
}
and from any part of your application:
SharedData.jsonData = new ArrayList<Custom>();
and to read the data
Custom obj = SharedData.jsonData.get(0);
Singleton wiki
Now, I want to share the value all the classes in application. So, what is recommended and efficient way to do that??
Assuming that you missed out a preposition "with" in your above question and want to use your arraylist in all other classes as well, you might wanna go for a global Arraylist, which is nothing but declaring a public static Arraylist in a new class (let's call it GlobalState.java).
/** GLobalState.java **/
public class GlobalState {
public static ArrayList<E> jsonGlobalArrayList;
}
You can use getters & setters if you want.
Now, u can assign your arraylist to this global Arraylist as soon as u have populated your arraylist after parsing the json file.
GlobalState.jsonGlobalArrayList = jsonArrayList;
This is how I do it whenever I'm dealing with json values. Hope this helps.
P.S.: Excuse me for oversimplifying the solution this elaborately, as u might have understood this already when I mentioned about global ArrayList. :)
public class Utilities {
public static boolean boolVar = false ;
public static int intVar= 0 ;
public static String str = "" ;
public static List<Object> aList= new ArrayList<Object>() ;
public static HashMap<String, Object> aMap= new HashMap<String, Object>() ;
}
Use them by using class name like;
Utilities.boolVar = boolValue ; Utilities.intVar = intValue ;
inside activities.