REPORT ztest.
PARAMETERS: clear TYPE char1 as CHECKBOX DEFAULT abap_true,
gc TYPE char1 as CHECKBOX DEFAULT abap_true.
CLASS lcl_person DEFINITION.
PUBLIC SECTION.
DATA: mv_name TYPE string.
METHODS: constructor IMPORTING !iv_name TYPE string.
ENDCLASS.
CLASS lcl_person IMPLEMENTATION.
METHOD: constructor.
me->mv_name = iv_name.
ENDMETHOD.
ENDCLASS.
START-OF-SELECTION.
DATA: lo_person TYPE REF TO lcl_person,
lo_weak TYPE REF TO cl_abap_weak_reference.
lo_person = NEW lcl_person( 'Jerry' ).
lo_weak = NEW cl_abap_weak_reference( lo_person ).
IF clear = abap_true.
CLEAR: lo_person.
ENDIF.
IF gc = abap_true.
cl_abap_memory_utilities=>do_garbage_collection( ).
ENDIF.
lo_person = CAST lcl_person( lo_weak->get( ) ).
IF lo_person IS INITIAL.
WRITE: / 'reference not available'.
ELSE.
WRITE: / 'reference still available'.
ENDIF.

import java.lang.ref.WeakReference;
class Person {
  private String mName;
  public Person(String name) {
    this.mName = name;
  }
  public String getName() {
    return this.mName;
  }
}
public class WeakReferenceTest {
  public static void check(Person person) {
    if (person == null) {
      System.out.println("Reference invalid");
    }
    else {
      System.out.println("Reference still available");
    }
  }
  public static void main(String[] args) {
    Person jerry = null;
    WeakReference<Person> person = new WeakReference<Person>(new Person(
        "Jerry"));
    jerry = new Person("Ben");
    // if you comment out this line, Reference will be available
    System.gc();
    Person restore = person.get();
    check(restore);
  }
}

package reference;
import java.lang.ref.SoftReference;
import java.util.ArrayList;
class Person2 {
  private String mName;
  public Person2(String name) {
    this.mName = name;
  }
  public String getName() {
    return this.mName;
  }
  public void finalize() {
    System.out.println("finalize called: " + this.mName);
  }
  public String toString() {
    return "Hello, I am " + this.mName;
  }
}
public class SoftReferenceTest {
  public static void main(String[] args) {
    SoftReference<Person2> person = new SoftReference<Person2>(new Person2(
        "Jerry"));
    System.out.println(person.get());
    ArrayList<Person2> big = new ArrayList<Person2>();
    for (int i = 0; i < 10000; i++) {
      big.add(new Person2(String.valueOf(i)));
    }
    System.gc();
    System.out.println("End: " + person.get());
  }
}

package aop;
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
public class PhantomReferenceTest {
  public static void main(String[] args) {
    Object phantomObj;
    PhantomReference phantomRef, phantomRef2;
    ReferenceQueue phantomQueue;
    phantomObj = new String("Phantom Reference");
    phantomQueue = new ReferenceQueue();
    phantomRef = new PhantomReference(phantomObj, phantomQueue);
    System.out.println("1 Phantom Reference:" + phantomRef.get());
    System.out.println("2 Phantom Queued: " + phantomRef.isEnqueued());
    phantomObj = null;
    System.gc();
    System.out.println("3 Anything in Queue? : " + phantomQueue.poll());
    if (!phantomRef.isEnqueued()) {
      System.out.println("4 Requestion finalization.");
      System.runFinalization();
    }
    System.out.println("5 Anything in Queue?: " + phantomRef.isEnqueued());
    phantomRef2 = (PhantomReference) phantomQueue.poll();
    System.out.println("6 Original PhantomReference: " + phantomRef);
    System.out.println("7 PhantomReference from Queue: " + phantomRef2);
  }
}