Does Java have lazy evaluation?

Solution 1:

Well, as far as the language is concerned - yes, both functions are called.

If you rewrote the function to this:

public boolean isTrue() {
    return isBTrue() || isATrue();
}

then the second function will not be called, if the first is true.

But this is short-circuit evaluation, not lazy evaluation. Lazy evaluation case would look something like this:

public interface LazyBoolean {
    boolean eval();
}

class CostlyComparison implements LazyBoolean {
  private int a, b;

  public CostlyComparison(int a, int b) { 
    this.a=a; 
    this.b=b; 
  }

  @Override 
  public boolean eval() {
    //lots of probably not-always-necessary computation here
    return a > b;
  }
} 

public LazyBoolean isATrue() {
  return new CostlyComparison(10,30);  //just an example
}

public boolean isTrue() {        // so now we only pay for creation of 2 objects
    LazyBoolean a = isATrue();   // but the computation is not performed; 
    LazyBoolean b = isBTrue();   // instead, it's encapsulated in a LazyBoolean
    return b.eval() || a.eval(); // and will be evaluated on demand;
                                 // this is the definition of lazy eval.
}

Solution 2:

In Java (and other C-like languages), this is referred to as short-circuit evaluation.^*

And yes, in the second example isATrue is always called. That is, unless the compiler/JVM can determine that it has no observable side-effects, in which case it may choose to optimize, but in which case you wouldn't notice the difference anyway.