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· java,ocp

Given the following code fragment:

00: ExecutorService es = ...

01: // es.submit(() -〉 {;} );

02: // es.submit(() -〉 null );

03: // es.submit(() -〉 { throw new NullPointerException(); });

04: // es.submit(() -〉 { throw new IOException(); });

05: // es.submit(() -〉 new SQLException());

Which line or lines, when uncommented individually, will compile successfully? Choose one.

  • * Only line 01
  • * Only lines 01 and 02
  • * Only lines 01, 02, and 03
  • * Only lines 01, 02, 03, and 04
  • * All lines will compile successfully

#java #certificationquestion #ocp

Answer. The java.util.concurrent.ExecutorService has three overloaded submit(...) methods.

* 〈T〉 Future〈T〉 submit(Callable〈T〉 task);

* Future〈?〉 submit(Runnable task);

* 〈T〉 Future〈T〉 submit(Runnable task, T result);

In each case, the task is defined by a particular method on the argument object, and that task-defining method is declared in the interface Callable or Runnable.

Those interfaces have the following forms:

public interface Runnable {

public abstract void run();


public interface Callable〈V〉 {

V call() throws Exception;


Notice that there are two significant differences between them.

* A Callable returns a value, whereas the Runnable declares a void method.

* A Callable may throw a checked exception, but a Runnable can throw only an unchecked exception.

Now, let's check the suggestions one by one:

Line 01: () -〉 {;}

This lambda body does not return any value, so it can implement only Runnable.

Line 01 is valid and will compile.

Line 02: () -〉 null

This form creates a Callable because it returns a value.

Line 02 is also valid.

In lines 03 and 04, the lambda has a body that consistently throws an exception.

Runnable can throw only unchecked exceptions, but a Callable may throw any exception.

This means that line 03 could be either a Runnable or a Callable, but line 04 must be a Callable.

Both lines are valid and will compile.

Line 05: () -〉 new SQLException()

It returns an exception, rather than throwing an exception.

However, exceptions are objects, so the code forms a Callable because it returns a value.

Line 05 is valid and will compile.

All five lambdas are valid.