Question

Why I cannot put a breakpoint on line String a;?

public void localMethod() {
    String a;
    a = "haha";
    System.out.println(a);
}

I know local variable will not be initialized until I assign a value to it explicitly.
But it is a line of code, it does something. Why I cannot stop there?
What lines are eligible to be breakpoints?

I am using Eclipse, jdk6_31

Answer 1

The declaration itself isn’t really executable code – it’s just declaring the existence of something. You should be able to put a breakpoint on the second line, which actually does something.

Admittedly I don’t see any reason why an IDE shouldn’t support the notion of adding a breakpoint to a non-executable line – it would probably have to really install the breakpoint at an executable point within the executing environment…

EDIT: To clarify what I mean, this code:

public void foo() {
    String a;
    String b;
    a = "hello";
    b = " world";
    System.out.println(a + b);        
}

will compile to the same bytecode as:

public void foo() {
    String a;
    a = "hello";
    String b;
    b = " world";
    System.out.println(a + b);        
}

No code has to execute due to a declaration – it doesn’t reserve space at that point in time or anything like that. The compiler allocates a “slot” within the method’s stack space, and will use that slot throughout the method – but it can reuse that same slot with no extra initialization even if the variable is declared within a loop, for example.