Default value of 'boolean' and 'Boolean' in Java

What are the default values of 开发者_如何学Pythonboolean (primitive) and Boolean (primitive wrapper) in Java?

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The default value for a Boolean (object) is null.
The default value for a boolean (primitive) is false.

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The default value of any Object, such as Boolean, is null.

The default value for a boolean is false.

Note: Every primitive has a wrapper class. Every wrapper uses a reference which has a default of null. Primitives have different default values:

boolean -> false

byte, char, short, int, long -> 0

float, double -> 0.0

Note (2): void has a wrapper Void which also has a default of null and is it's only possible value (without using hacks).

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boolean
Can be true or false.
Default value is false.

(Source: Java Primitive Variables)

Boolean
Can be a Boolean object representing true or false, or can be null.
Default value is null.

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If you need to ask, then you need to explicitly initialize your fields/variables, because if you have to look it up, then chances are someone else needs to do that too.

The value for a primitive boolean is false as can be seen here.

As mentioned by others the value for a Boolean will be null by default.

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Boolean is an Object. So if it's an instance variable it will be null. If it's declared within a method you will have to initialize it, or there will be a compiler error.

If you declare as a primitive i.e. boolean. The value will be false by default if it's an instance variable (or class variable). If it's declared within a method you will still have to initialize it to either true or false, or there will be a compiler error.

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An uninitialized Boolean member (actually a reference to an object of type Boolean) will have the default value of null.

An uninitialized boolean (primitive) member will have the default value of false.

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There is no default for Boolean. Boolean must be constructed with a boolean or a String. If the object is unintialized, it would point to null.

The default value of primitive boolean is false.

http://download.oracle.com/javase/6/docs/api/java/lang/Boolean.html
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

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class BooleanTester
{
    boolean primitive;
    Boolean object;

    public static void main(String[] args) {
        BooleanTester booleanTester = new BooleanTester();
        System.out.println("primitive: " + booleanTester.getPrimitive());
        System.out.println("object: " + booleanTester.getObject());
}

    public boolean getPrimitive() {
        return primitive;
    }

    public Boolean getObject() {
        return object;
    }
}

output:

primitive: false
object: null

This seems obvious but I had a situation where Jackson, while serializing an object to JSON, was throwing an NPE after calling a getter, just like this one, that returns a primitive boolean which was not assigned. This led me to believe that Jackson was receiving a null and trying to call a method on it, hence the NPE. I was wrong.

Moral of the story is that when Java allocates memory for a primitive, that memory has a value even if not initialized, which Java equates to false for a boolean. By contrast, when allocating memory for an uninitialized complex object like a Boolean, it allocates only space for a reference to that object, not the object itself - there is no object in memory to refer to - so resolving that reference results in null.

I think that strictly speaking, "defaults to false" is a little off the mark. I think Java does not allocate the memory and assign it a value of false until it is explicitly set; I think Java allocates the memory and whatever value that memory happens to have is the same as the value of 'false'. But for practical purpose they are the same thing.