public class Gen<T> {

  private T t;

  public Gen(T t){
    this.t = t;
  }
  public T getT() {
    return t;
  }
  public void setT(T t) {
    this.t = t;
  }

  public void showType(){
    System.out.println("T的实际类型是：" + t.getClass().getName());
  }

  public static void main(String[] args) {
    Gen<Integer> gen = new Gen<Integer>(1);
    gen.showType();

    int i = gen.getT(); 
    System.out.println(" value = " + i); 
    System.out.println(" ====================== "); 

    //定义泛型类Gen的一个String的版本 
    Gen<String>strObj = new Gen<String>("Hello Gen!"); 
    strObj.showType(); 
    String s = strObj.getT(); 
    System.out.println(" value = " + s); 

  }
}

public class Gen2 { 

  // 定义一个通用类型成员 
  private Object obj; 

  public Gen2(Object obj) { 
    this.obj = obj; 
  } 

  public Object getObj() { 
    return obj; 
  } 

  public void setObj(Object obj) { 
    this.obj = obj; 
  } 

  public void showType() { 
    System.out.println("T的实际类型是: " + obj.getClass().getName()); 
  } 

  public static void main(String[] args) { 
    // 定义类Gen2的一个Integer版本 
    Gen2 intObj = new Gen2(2); 
    intObj.showType(); 

    int i = (Integer) intObj.getObj(); 
    System.out.println(" value = " + i); 
    System.out.println(" ====================== "); 

    // 定义类Gen2的一个String版本 
    Gen2 strOb = new Gen2("Hello Gen!"); 
    strOb.showType(); 
    String s = (String) strOb.getObj(); 
    System.out.println(" value= " + s); 
  } 
} 

GenericsFoo<Double> douFoo=new GenericsFoo<Double>(new Double("33"));

GenericsFoo douFoo=new GenericsFoo(new Double("33"));

<T extends SomeClass & interface1 & interface2 & interface3>

public class Demo<T extends Comparable & Serializable> { 
  // T类型就可以用Comparable声明的方法和Seriablizable所拥有的特性了 
} 

public class GenericMethod { 

  public <T> void print(T x) { 
    System.out.println(x.getClass().getName()); 
  } 

  public static void main(String[] args) { 
    GenericMethod method = new GenericMethod(); 
    method.print(" "); 
    method.print(10); 
    method.print('a'); 
    method.print(method); 
  } 
} 

public void write(Integer i, Integer[] ia);
public void write(Double d, Double[] da);

public <T> void write(T t, T[] ta);

public class TestClassDefine<T, S extends T>{......}

public <T, S extends T> T testGenericMethodDefine(T t, S s){......}

List<String> list;
list = new ArrayList<String>;

public class MyList<E> extends ArrayList<E> implements List<E> {...} 

List<?> unknownList;
List<? extends Number> unknownNumberList;
List<? super Integer> unknownBaseLineIntgerList;