弱い参照とな ref: http://qiita.com/yyyske/items/daa5c844647604e27e4f

Main.java

package references;

import java.lang.ref.*;

public class Main {

    public static void main(String[] args) {

        Sheryl strongRef = new Sheryl();//①
        WeakReference<Sheryl> weakRef = new WeakReference<Sheryl>(strongRef);//②

        strongRef = null;//③

        Sheryl sheryl = weakRef.get();
        if (sheryl != null){
            System.out.println("まだ生きてます");
            sheryl.sing();
        }else{
            System.out.println("消えました");
        }
        sheryl = null;

        System.gc();//④

        System.out.println("GCしました");

        Sheryl sheryl = weakRef.get();
        if (sheryl != null){
            System.out.println("まだ生きてます");
            sheryl.get().sing();
        }else{
            System.out.println("消えました");
        }

    }

}

Sheryl.java

package references;

public class Sheryl {

    public void sing(){
        System.out.println("生き残りたい"); 
    }

}

結果

まだ生きてます
生き残りたい
GCしました
消えました