Related
This is always very confusing to me. Can someone please explain it? The confusion I have is - boolean default to false. So in the below example, does it enter the if loop when state is not turned on, i.e., it is TURNED OFF if (condition is false) OR does it enter the if loop when state is TURNED ON, in other words if (condition is true)?
boolean turnedOn;
if (turnedOn) {
//do stuff when the condition is false or true?
} else {
//do else of if
}
I know this is a very basic question, but if you could explain the answer in very basic language, that would be great. :) Feel free to point me to duplicate posts that have a very good explanation (I did not find one where I could clearly get it). Also feel free to change the subject of the post if you'd like to make it more generic.
Okay, so..
// As you already stated, you know that a boolean defaults to false.
boolean turnedOn;
if(turnedOn) // Here, you are saying "if turnedOn (is true, that's implicit)
{
//then do this
}
else // if it is NOT true (it is false)
{
//do this
}
Does it make more sense now?
The if statement will evaluate whatever code you put in it that returns a boolean value, and if the evaluation returns true, you enter the first block. Else (if the value is not true, it will be false, because a boolean can either be true or false) it will enter the - yep, you guessed it - the else {} block.
A more verbose example.
If I am asked "are you hungry?", the simple answer is yes (true). or no (false).
boolean isHungry = true; // I am always hungry dammit.
if(isHungry) { // Yes, I am hungry.
// Well, you should go grab a bite to eat then!
} else { // No, not really.
// Ah, good for you. More food for me!
// As if this would ever happen - bad example, sorry. ;)
}
In your example, the IF statement will run when it is state = true meaning the else part will run when state = false.
if(turnedOn == true) is the same as if(turnedOn)
if(turnedOn == false) is the same as if(!turnedOn)
If you have:
boolean turnedOn = false;
Or
boolean turnedOn;
Then
if(turnedOn)
{
}
else
{
// This would run!
}
ABoolean (with a uppercase 'B') is a Boolean object, which if not assigned a value, will default to null. boolean (with a lowercase 'b') is a boolean primitive, which if not assigned a value, will default to false.
Boolean objectBoolean;
boolean primitiveBoolean;
System.out.println(objectBoolean); // will print 'null'
System.out.println(primitiveBoolean); // will print 'false'
Citation
so in your code because boolean with small 'b' is declared it will set to false hence
boolean turnedOn;
if(turnedOn) **meaning true**
{
//do stuff when the condition is false or true?
}
else
{
//do else of if ** itwill do this part bechae it is false
}
the if(turnedon) tests a value if true, you didnt assign a value for turned on making it false, making it do the else statement :)
boolean turnedOn;
if(turnedOn)
{
//do stuff when the condition is true - i.e, turnedOn is true
}
else
{
//do stuff when the condition is false - i.e, turnedOn is false
}
boolean state = "TURNED ON";
is not a Java valid code. boolean can receive only boolean values (true or false) and "TURNED ON"is a String.
EDIT:
now you are talking about a loop and your code does not contain any. your var state is false because the boolean default value and you execute the else clause.
Suppose you want to check a boolean. If true, do something. Else, do something else. You can write:
if(condition==true){
}
else{ //else means this checks for the opposite of what you checked at if
}
instead of that, you can do it simply like:
if(condition){ //this will check if condition is true
}
else{
}
Inversely. If you were to do something if condition was false and do something else if condition was true. Then you would write:
if(condition!=true){ //if(condition=false)
}
else{
}
But following the simple path. We do:
if(!condition){ //it reads out as: if condition is not true. Which means if condition is false right?
}
else{
}
Think about it. You'll get it in no time.
Booleans default value is false only for classes' fields. If within a method, you have to initialize your variable by true or false. Thus for example in your case, you'll have a compilation error.
Moreover, I don't really get the point, but the only way to enter within a if is to evaluate the condition to true.
Assuming state is having a valid boolean value set in your actual code, then the following condition will succeed
if(state)
when state is boolean value is TRUE
If condition checks for the expression whether it is evaluated to TRUE/FALSE. If the expression is simple true then the condition will succeed.
This is how the if behaves.
if(turnedOn) // This turnedOn should be a boolean or you could have a condition here which would give a boolean result.
{
// It will come here if turnedOn is true (i.e) the condition in the "if" evaluates to true
}
else
{
// It will come here if turnedOn is false (i.e) the condition in the "if" evaluates to false
}
The syntax of if block is as below,
if(condition){
// Executes when condition evaluates to true.
}
else{
// Executes when condition evaluates to false.
}
In your case you are directly passing a boolean value so no evaluation is required.
if (turnedOn) {
//do stuff when the condition is false or true?
}
else {
//do else of if
}
It can be written like:
if (turnedOn == true) {
//do stuff when the condition is false or true?
}
else { // turnedOn == false or !turnedOn
//do else of if
}
So if your turnedOn variable is true, if will be called, if is assigned to false, else will be called. boolean values are implicitly assigned to false if you won't assign them explicitly e.q. turnedOn = true
Every time the condition "if (turnedOn)", always refers as "TRUE condition", unless the condition is "if (!turnedOn)" will refer as "FALSE condition".
In other case, if you want to compare two Boolean condition, for example;
Two boolean variable: turnedOn, switchedOn
Let's say current condition;
turnedOn=true
switchedOn=false
"if (turnedOn) || if (switchedOn)" will return TRUE
"if (turnedOn) && if (switchedOn)" will return FALSE
this is my first so I'll try to add as much info as possible so I don't get yelled at. :-)
What I am trying to do is I have 2 variables that grab text from 2 fields and take only the first character from each and assign it to those values.
This is the code that I use to get the strings. They are 2 separate calls as you would.
try { var_ContactSurname = var_ContactSurname.substring(0,1);
}
catch (Exception e){
}
I have the above again with a different variable. Now to this point it does what I want. It grabs the first letter from the fields and assigns it to the variables.
So at this point I have two variables (say with an example charater of D and R).
var_ContactSurname = R
var_ContactLicenceNumber = D
What I want to do is compare those two variables and if they match I want to return a value of TRUE, else FALSE if they don't match.
That value has to be a string as well and be assigned to a new variable called var_ContactValidate.
if (var_ContactLicenceNumber.toLowerCase().equals()var_ContactSurname.toLowerCase()){
var_ContactValidate == "TRUE";
}
else {
var_ContactValidate == "FALSE";
}
No you may notice that there might be some code missing. I am using a rules engine that does a lot of the functions for me. I can use raw Java code to do other things (like this compare)...but that's the compare that I am having a problem with.
Any ideas for that compare would be greatly appreciated.
Thanks.
i would use the String method equalsIgnoreCase()
to assign a value to a field, use a single =, not double (==).
if (var_ContactLicenceNumber.equalsIgnoreCase(var_ContactSurname){
var_ContactValidate = "TRUE";
}
else {
var_ContactValidate = "FALSE";
}
check it
In addition to what already said - a simpler & more elegant version (without the if condition) could be:
var_ContactValidate = Boolean.toString(
var_ContactLicenceNumber.equalsIgnoreCase(var_ContactSurname))
.toUpperCase();
Change your whole piece of code to:
if (var_ContactLicenceNumber.equalsIgnoreCase(var_ContactSurname)){
var_ContactValidate == "TRUE";
}
else {
var_ContactValidate == "FALSE";
}
This combines the case insensitivity that you want, and passes through the second string as an argument of the .equalsIgnoreCase function.
Also, I am not sure what you are trying to do with the line:
var_ContactValidate == "TRUE";
If you want to assign var_ContactValidate to "TRUE" then use a single equals sign '=' as a double equals '==' compares the values instead. You may also considering using a boolean rather than a string in this case.
Here is an implementation that also checks for null values and empty Strings:
public class SurnameAndLicenseValidator {
public static void main(String[] args) {
// FALSE
validateSurnameAndLicense(null, "jb78hq");
validateSurnameAndLicense("Johnson", null);
validateSurnameAndLicense(null, null);
validateSurnameAndLicense("", "jb78hq");
validateSurnameAndLicense("Johnson", "");
validateSurnameAndLicense("", "");
validateSurnameAndLicense("johnson", "xb78hq");
// TRUE
validateSurnameAndLicense("Johnson", "jb78hq");
validateSurnameAndLicense("johnson", "jb78hq");
}
private static String validateSurnameAndLicense(String surname,
String license) {
String result;
if (surname != null
&& surname.length() > 0
&& license != null
&& license.length() > 0
&& Character.toUpperCase(surname.charAt(0)) == Character
.toUpperCase(license.charAt(0))) {
result = "TRUE";
} else {
result = "FALSE";
}
System.out.println(surname + " " + license + " " + result);
return result;
}
}
The main method is used as a unit test here. You might want to extract a real JUnit test from it, if you are into that kind of thing.
In a program I am trying to check two boolean values (returned from a function); the condition that needs to be checked is:
- only if any one of the returned value is true and the other is false then I have a problem;
- else if both are true or false I am good to go to next step.
Which of the following two examples would be the efficient way to check the condition, or is there a better solution?
a and b are integer values on which I am checking a condition for correctness in isCorrect function and it return true or false.
1.
// checking for the correctness of both a and b
if ((isCorrect(a) && !isCorrect(b)) ||
(!isCorrect(a) && isCorrect(b)))
{
// a OR b is incorrect
}
else
{
// a AND b are both correct or incorrect
}
2.
// checking for the correctness of both a and b
if (! (isCorrect(a) ^ isCorrect(b)))
{
// a OR b is incorrect
}
else
{
// a AND b are correct or incorrect
}
Thanks,
Ivar
P.S: code readability is not an issue.
EDIT: I meant to have an XOR in the second option.
Also, I agree with the == and != options, but what if I had to use boolean operators?
if (isCorrect(a) != isCorrect(b)) {
// a OR b is incorrect
} else {
// a AND b are correct or incorrect
}
Your test doesn't need boolean operators, just this:
if (isCorrect(a) == isCorrect(b)) {
// they both have the same value
} else {
// they don't ...
}
EDIT - I deliberately didn't use the same comments to reflect that the primary purpose of the comment should be to describe the intent, and not the specific implementation. In this case the simplest statement of intent is that both a and b obtained the same result.
simply:
if (isCorrect(a) == isCorrect(b))
{
// a AND b are both correct or incorrect
} else {
// a OR b is incorrect
}
How about this?
if(isCorrect(a) != isCorrect(b))
{
//problem
}
else
{
//not a problem
}
You can use XOR also, but != works fine and is more readable if you are dealing with boolean values, IMO.
In my program below, I set the variable th as true in the second if statement.
I'm curious why it later returns as false.
public boolean nodeExist(TreeNode Tree, T value){
boolean th = false;
if(Tree.getValue()!= null){
if(value == Tree.getValue()){
th = true;
}else{
if(value.compareTo((T) Tree.getValue()) < 0){
nodeExist(Tree.getLeft(), value);
}else{
nodeExist(Tree.getRight(), value);
}
}
}else{
th = false;
}
return th;
}
You probably look at a recursive call which sets th to true. But when that call returns to its caller, that th is still at false, and that's then returned. You need to assign the recursive callee's result:
if(value.compareTo((T) Tree.getValue()) < 0){
th = nodeExist(Tree.getLeft(), value);
}else{
th = nodeExist(Tree.getRight(), value);
}
You already got your answer. In future, to prevent mistakes like this, it's better to just return the value right away if you can. IT'S OKAY to have multiple return; if used properly, it can read to more readable code.
public boolean nodeExist(TreeNode Tree, T value){
if (Tree.getValue() == null) return false;
if (value == Tree.getValue()) return true;
if (value.compareTo((T) Tree.getValue()) < 0) {
return nodeExist(Tree.getLeft(), value);
} else {
return nodeExist(Tree.getRight(), value);
}
}
See also
Should a function have only one return statement?
Additionally, I noticed that you used == instead of equals for object comparison (i.e. T can't be a primitive type). This is rarely correct; equals is almost always what is really intended.
See also
Difference Between Equals and ==
One more style advice, please follow naming convention for Java, where variable names start with lowercase letter, with upper case letter for internal words (so, somethingLikeThis).
Programming isn't about getting things right, it's also about getting things readable. Learn and adopt a good coding style, and follow established conventions.
In the section in which you're doing your compareTo where the th value is not set. If this conditional is met, th can never be set to true.
I did a little search on this but couldn't find anything useful.
The point being that if String value is either "true" or "false" the return value should be true. In every other value it should be false.
I tried these:
String value = "false";
System.out.println("test1: " + Boolean.parseBoolean(value));
System.out.println("test2: " + Boolean.valueOf(value));
System.out.println("test3: " + Boolean.getBoolean(value));
All functions returned false :(
parseBoolean(String) returns true if the String is (case-insensitive) "true", otherwise false
valueOf(String) ditto, returns the canonical Boolean Objects
getBoolean(String) is a red herring; it fetches the System property of the given name and compares that to "true"
There exists no method to test whether a String encodes a Boolean; for all practical effects, any non-"true"-String is "false".
return "true".equals(value) || "false".equals(value);
Apache commons-lang3 has BooleanUtils with a method toBooleanObject:
BooleanUtils.toBooleanObject(String str)
// where:
BooleanUtils.toBooleanObject(null) = null
BooleanUtils.toBooleanObject("true") = Boolean.TRUE
BooleanUtils.toBooleanObject("false") = Boolean.FALSE
BooleanUtils.toBooleanObject("on") = Boolean.TRUE
BooleanUtils.toBooleanObject("ON") = Boolean.TRUE
BooleanUtils.toBooleanObject("off") = Boolean.FALSE
BooleanUtils.toBooleanObject("oFf") = Boolean.FALSE
BooleanUtils.toBooleanObject("blue") = null
if ("true".equals(value) || "false".equals(value)) {
// do something
} else {
// do something else
}
Here's a method you can use to check if a value is a boolean:
boolean isBoolean(String value) {
return value != null && Arrays.stream(new String[]{"true", "false", "1", "0"})
.anyMatch(b -> b.equalsIgnoreCase(value));
}
Examples of using it:
System.out.println(isBoolean(null)); //false
System.out.println(isBoolean("")); //false
System.out.println(isBoolean("true")); //true
System.out.println(isBoolean("fALsE")); //true
System.out.println(isBoolean("asdf")); //false
System.out.println(isBoolean("01truefalse")); //false
The methods you're calling on the Boolean class don't check whether the string contains a valid boolean value, but they return the boolean value that represents the contents of the string: put "true" in string, they return true, put "false" in string, they return false.
You can surely use these methods, however, to check for valid boolean values, as I'd expect them to throw an exception if the string contains "hello" or something not boolean.
Wrap that in a Method ContainsBoolString and you're go.
EDIT
By the way, in C# there are methods like bool Int32.TryParse(string x, out int i) that perform the check whether the content can be parsed and then return the parsed result.
int i;
if (Int32.TryParse("Hello", out i))
// Hello is an int and its value is in i
else
// Hello is not an int
Benchmarks indicate they are way faster than the following:
int i;
try
{
i = Int32.Parse("Hello");
// Hello is an int and its value is in i
}
catch
{
// Hello is not an int
}
Maybe there are similar methods in Java? It's been a while since I've used Java...
Actually, checking for a Boolean type in a String (which is a type) is impossible. Basically you're asking how to do a 'string compare'.
Like others stated. You need to define when you want to return "true" or "false" (under what conditions). Do you want it to be case(in)sensitive? What if the value is null?
I think Boolean.valueOf() is your friend, javadoc says:
Returns a Boolean with a value represented by the specified String. The Boolean returned represents the value true if the string argument is not null and is equal, ignoring case, to the string "true".
Example: Boolean.valueOf("True") returns true.
Example: Boolean.valueOf("yes") returns false.
Can also do it by regex:
Pattern queryLangPattern = Pattern.compile("true|false", Pattern.CASE_INSENSITIVE);
Matcher matcher = queryLangPattern.matcher(booleanParam);
return matcher.matches();
Yes, but, didn't you parse "false"? If you parse "true", then they return true.
Maybe there's a misunderstanding: the methods don't test, if the String content represents a boolean value, they evaluate the String content to boolean.
String value = "True";
boolean result = value.equalsIgnoreCase("true") ? true : false;
Well for this, also have a look at org.apache.commons.lang.BooleanUtils#toBoolean(java.lang.String), along with many other useful functions.
return value.equals("false") || value.equals("true");
Something you should also take into consideration is character casing...
Instead of:
return value.equals("false") || value.equals("true");
Do this:
return value.equalsIgnoreCase("false") || value.equalsIgnoreCase("true");
I suggest that you take a look at the Java docs for these methods. It appears that you are using them incorrectly. These methods will not tell you if the string is a valid boolean value, but instead they return a boolean, set to true or false, based on the string that you pass in, "true" or "false".
http://www.j2ee.me/javase/6/docs/api/java/lang/Boolean.html
See oracle docs
public static boolean parseBoolean(String s) {
return ((s != null) && s.equalsIgnoreCase("true"));
}
function isBooleanString(val) {
if (val === "true" || val === "false"){
return true
} else {
return false
}
}
isBooleanString("true") // true
isBooleanString("false") // true
isBooleanString("blabla") // false