Does Java have lazy evaluation?

Question

I know that Java has smart/lazy evaluation in this case:

public boolean isTrue() {     boolean a = false;     boolean b = true;     return b || (a && b); // (a && b) is not evaluated since b is true } 

But what about:

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

Is isATrue() called even if isBTrue() returns true?

Answer 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.

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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. }