Default Values - Java Clarification

Question

Why aren't default values assigned to the variables, that haven't been initialized within a Class with main function???

class Test {
    public static void main(String[] args) {

        int x;// x has no default value
        String y;// y has no default value
        System.out.println("x is " + );
        System.out.println("y is " + );
    }
}

Whereas the default values get assigned in case the variables remain uninitialized in a class without any main function.

class Student {
    String name; // name has default value null
    int age; // age has default value 0
    boolean isScienceMajor; // isScienceMajor has default value false
    char gender; // c has default value '\u0000'
    int x;
    String y;
}

You're misunderstanding the situation. It's not that one class has a main function and the other doesn't. It's that in the first example, the variables are declared *inside* the function (they're local variables) and in the second, they're declared *outside* the function, at the class level (they're *fields* or *member variables*). — Sebastian Redl, Jul 25 '13 at 13:55

score 5 · Answer 1 · edited Jun 20 '20 at 09:12

Be aware that the code in the question represents different situations. In the first case, the variables are local and exist inside the main() method. In the second case you're declaring instance attributes, not local variables.

In Java, only the attributes are initialized automatically with default values. In all the methods, even in the main() method, you must explicitly provide an initialization value for the variables declared locally inside the method.

This is explained in the Java Language Specification, section §4.12.5:

Each class variable, instance variable, or array component is initialized with a default value when it is created (§15.9, §15.10)

Each method parameter (§8.4.1) is initialized to the corresponding argument value provided by the invoker of the method (§15.12)

Each constructor parameter (§8.8.1) is initialized to the corresponding argument value provided by a class instance creation expression (§15.9) or explicit constructor invocation (§8.8.7)

An exception parameter (§14.20) is initialized to the thrown object representing the exception (§11.3, §14.18)

A local variable (§14.4, §14.14) must be explicitly given a value before it is used, by either initialization (§14.4) or assignment (§15.26), in a way that can be verified using the rules for definite assignment (§16)

To see several reasons why local variables are not initialized automatically, please take a look at this previous question.

score 2 · Answer 2 · answered Jul 25 '13 at 13:55

The basic reason is that, in order to catch a common programmer error, the Java authors decided to ensure that every variable is assigned before it is used. However, it is not possible to enforce this for fields, so they had to spec default values for fields.

You do get an error if you use a field in a constructor before it is initialized, but there is no way for the compiler to prevent this:

class C {
  int f;

  C() {
    init();
  }

  void init() {
    System.out.println(f);
  }
}

Joni · Answer 3 · 2013-07-25T19:16:34.620

Reading the value of a variable before it has been given a value is a common source of bugs. Forcing you to assign a value before using a variable makes programs safer: you'll know you'll get the value you expect and not some default value just because you didn't anticipate a certain program flow.

Unfortunately the liveness analysis that implementing this needs can only be done for local variables, and you can access instance fields before the program has initialized them with a value. To avoid unpredictable behavior the JVM initializes instance fields to default values.

score 1 · Answer 4 · answered Jul 25 '13 at 13:56

1

When the JVM is creating the new object instance, it has to allocate memory for the attributes because they are part of the class itself. The mere existence of a Java primitive int requires 4 bytes of memory to be allocated, whereas an Integer can be set to null since it's an object. That's why classes must have their attributes set to something upon being initialized.

answered Jul 25 '13 at 13:56

Mike Thomsen

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"whereas an Integer can be set to null since it's an object?" Could u kindly clarify this part? Are u saying that the datatype "Integer" is an object? – RDPD Jul 25 '13 at 14:07
Yes. There is a class called java.lang.Integer that is an object wrapper for a standard int. – Mike Thomsen Jul 25 '13 at 14:56

Default Values - Java Clarification

4 Answers4