Java Notes

'if' Statement - if inside if

if inside if

You can put an if statement inside another if statement.

Nearest 'else'

If you use braces, there is no problem with deciding which else goes with which if For example,

if (age < 24) {
    if (height > 200) {
        c = Color.RED;
    }
} else {
    c = Color.BLUE;
}

Because the true and false parts are both single statements, you might be tempted to omit braces and write:

if (age < 24) 
    if (height > 200) 
        c = Color.RED;
else                // DANGER: which 'if' goes with this 'else'
    c = Color.BLUE;

But this is WRONG, because 'else' always goes with the nearest 'if' when there are no braces. This code is the same as:

if (age < 24) {
    if (height > 200) 
        c = Color.RED;
    else
        c = Color.BLUE;
}

Advice: Always use braces on if statements

These kinds of errors are very hard to find. This is another good reason to always use braces.

Watch out for semicolons on your if statements

Why does the following code always say it thinks the user is lying?

String ageStr = JOptionPane.showInputDialog(null, "How old are you?");
int age = Integer.parseInt(ageStr);
if (age > 120 || age < 0);
    System.out.println("I think you're lying about your age!");

It's the semicolon! if you put a semicolon directly after the condition in an if statement, Java thinks it's finished with the body of the statement. The indentation of the next line, which is so important to human readers, is ignored by Java.

This is another error that's harder to make if you always follow the condition by an opening brace.