Initialization of local variable in a conditional block in Java

Java comprises 5 conditional blocks namely – if, switch, while, for and try.
In all these blocks, if the specified condition is true, the code inside the block is executed and vice-versa. Also, Java compiler doesn’t let you leave a local variable uninitialized.

While initializing local variable inside a conditional block, one must bear the following 3 details in mind:
1. If the specified condition is true, and ‘values’ are provided in the condition, the program compiles fine.
2. If the specified condition is true, but ‘variables’ are provided in the condition, we get a compilation error.
3. If the specified condition is false, we get a compilation error.

The above-given points are true for both primitive and reference type local variables.

Example:
The code below will give a compilation error.

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// Java program to demonstrate error if we
// assign value to an uninitialized variable
// only in if block.
public class InitTesting {
    public static void main(String args[])
    {
        int i = 100;
        int j;
  
        // Note that the condition is false
        if (i > 500
            j = i + 5;
          
        System.out.println("j :" + j);
    }
}

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 Output: 
prog.java:8: error: variable j might not have been initialized
        System.out.println("j :" + j);
                                   ^
1 error

The condition in the if statement is false. Hence, local variable ‘j’ never gets initialized. Thus, trying to reference uninitialized variable ‘j’ in line8 gives a compilation error.
To avoid this, initialize your local variable to a default value outside the conditional block.
The code below works fine-

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// Java program to demonstrate that the above
// error is gone if we initialize the variable.
public class InitTesting {
    public static void main(String args[])
    {
        int i = 100;
        int j = 0;
  
        // Note that the condition is false
        if (i > 500) {
            j = i + 5;
        }
        System.out.println("j :" + j);
    }
}

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 Output:
j :0

Also, in Java, ‘values’ are read at compile time. But, ‘variables’ are read at run-time. Hence, when variables are a part of the condition, and another variable is initialized inside the conditional block, it gives an unexpected compile-time error.

Example: Have a look at the code below:

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// Java program to demonstrate error even if
// condition is true.
  
class Test {
    public static void main(String args[])
    {
        int a = 90;
        int b = 80;
        int i;
  
        // The condition is true
        if (a > b) {
            i = a + 5;
        }
        System.out.println("i :" + i);
    }
}

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 Output:
prog.java:9: error: variable i might not have been initialized
        System.out.println("i :" + i);
                                   ^
1 error

It gives a compilation error irrespective of the condition being true or false because Java didn’t read variables at compile time and thus ‘i’ isn’t initialized.

On the other hand, this doesn’t happen if values are specified instead of variables.

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// Java program to demonstrate that there is
// no error if we constants in if condition.
class Test {
    public static void main(String args[])
    {
        int i;
        if (90 > 80) {
            i = a + 5;
        }
        System.out.println("i :" + i);
    }
}

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 Output:
i :95


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