java - How come not implementing some Methods are allowed in WindowAdapter? -

the windowadapter class of java defined abstract class , has many abstract methods, including:

windowclosing() windowclosed() windowactivated() 

all of these methods empty , java says class exists convenience not want create classes implementing windowlistener. because unlike windowlistener interface, windowadapter gives choice implement 1 of abstract methods defined in it.

for example if add below code class inherits window, make window closeable through 'x' button on upper right corner:

addwindowlistener(new windowadapter() {             public void windowclosing(windowevent we) {                 dispose();             }         }); 

however confuses me. first of happening here? creating inner class extends windowadapter? new keyword used create instance, not allowed instantiate abstract class. why new keyword here?

second, why away implementing 1 of abstract methods in windowadapter? in java if define abstract class:

public abstract class upperclassab {     public abstract void test();      public abstract int boa();  } 

and try use class, have used windowadapterabove:

upperclassab tester = new upperclassab() {             public void test() {                 system.out.println("mor");             }         }; 

i error, because not implementing abstract methods 1 of them. how can away implementing 1 of abstract methods in case of windowadapter? single case, , if not can imitate behavior?

although windowadapter abstract class, methods not abstract empty. not have implement them, can override them if want.

https://docs.oracle.com/javase/7/docs/api/java/awt/event/windowadapter.html

regarding new keyword: called anonymous class. create new class without name, , instantiate @ same time. new class not have name, extends windowadapter, overrides 1 method, , can instantiated @ location (because has no name)