如何使用lambda表达式向一个方法发送一个条件，以便它还没有被评估？

Question

I am trying send a condition statement (that has not been evaluated yet) as an argument to a method. I understand that in java8 , the lambda expression is the way to do it (effectively putting the condition inside a function, and sending the function).

// simple method inside a utilities class 
//    that does assertions if global debug parameter is true

public class MyUtils
  {
    public  static void do_assertions( boolean argb , String args )
      {
        if( BuildConfig.pvb_debuggable )
          { assert argb  , args ;
          }
      }
  }

---

// somewhere within app where a development-phase assertion is needed

public class some_class
  { 
    public void some_method( )
      { 
        EditText lvo_editText = (EditText) findViewById( "the_id" ) ;

        //assert lvo_editText != null; 
        //   ... replace this with a call to do_assertions
        MyUtils.do_assertions( () -> { lvo_editText != null ; }  , "bad EditText" );
      } 
  }

I have tried many variations of this setup. I get different errors each time :)

Answer 1

You were almost there, you can change your signature to receive a BooleanSupplier which will evaluate the condition only when calling getAsBoolean .

Here a simple example :

public class Test {

    public static void main(String args[]) {
        A a = new A();
        test(() -> a != null && a.get());
    }

    static void test(BooleanSupplier condition) {
        condition.getAsBoolean();
    }

    static class A {
        boolean get(){
            return true;
        }
    }
}

If you go through this example in debug mode, you'll see that the condition a != null && a.get() is evaluated only when condition.getAsBoolean() is executed.

Applying this to your example, you would only need to change

void do_assertions( boolean argb , String args )

for

void do_assertions(BooleanSupplier argo_supplier , String args )

and then call argo_supplier.getAsBoolean() where you want to evaluate the condition (after checking pvb_debuggable ).

Then your line

MyUtils.do_assertions( () -> lvo_editText != null  , "bad EditText" );

would compile properly (notice that I removed the unnecessary brackets).