Local variables in Lambdas vs Anonymous inner classes

Question

While going through the lambda expressions, I came across the below behavior for anonymous inner classes and lambda expressions. What could be the reason behind this?

Human h = new Human() {
int a = 2;
@Override
public void sing() {

System.out.println(++a);
}

};

h.sing();
h.sing();

O/P

3
4

Whereas for lambdas I get below:

Human h = () -> {

int a = 2;
System.out.println(++a);
};

h.sing();
h.sing();

}

O/P

3
3

The one in the anonymous class isn't a local variable, it's a field that stays alive as long as the object does. The one in the lambda is a local variable that only lives inside the method and isn't saved between calls. — user, Apr 25 '20 at 18:16
See https://stackoverflow.com/questions/48401216/can-java-lambdas-have-state/48401440 — Sotirios Delimanolis, Apr 25 '20 at 18:29

score 6 · Accepted Answer · answered Apr 25 '20 at 18:17

Those are not equivalent. The first function modifies a variable outside of it's scope, whereas in the second example each time you're invoking h.sing();, the body of that function is being invoked. That means, a variable is instantiated with value 2 every time.

score 5 · Answer 2 · answered Apr 25 '20 at 18:17

Your lambda is more or less equivalent to:

Human h = new Human() {
    @Override
    public void sing() {
        int a = 2;    
        System.out.println(++a);
    }
};
h.sing();
h.sing();

There is no way to declare a lambda with directly mutable state.

Local variables in Lambdas vs Anonymous inner classes

2 Answers2