Java 泛型 （Generics）

泛型：就是变量类型的参数化

泛型是JDK1.5中的一个最重要的特征。通过引入泛型，我们将获得编译时类型的安全和运行时更小的抛出ClassCastException的可能。

public class ArrayListTest {
	public static void main(String[] args) {
		List list = new ArrayList();
		list.add("Nick");
		list.add(new Integer(20));
		list.add(new Boolean(false));
		
		String str = (String) list.get(0);
		Integer in = (Integer)list.get(1);
		String b = (String)list.get(2);
		
	}
}

　　运行后，String b = (String)list.get(2);将抛出异常

Exception in thread "main" java.lang.ClassCastException: java.lang.Boolean cannot be cast to java.lang.String
	at com.example.fanxing.ArrayListTest.main(ArrayListTest.java:16)

　　

定义两个类，如下图

 以上两个类结构时一致的，如果在增加一种类型，如String，还需要创建StringDemo类。

可以优化成一个类统一处理：

public class ObjectDemo {

	private Object falg ;

	public Object getFalg() {
		return falg;
	}

	public void Integer(Object falg) {
		this.falg = falg;
	}
	
	
}

　　然后使用ObjectDemo

public static void main(String[] args) {
		ObjectDemo obj1 = new ObjectDemo();
		obj1.SetFlag("Nick");
		String falg = (String)obj1.getFalg();
		System.out.println("falg:" + falg);
		
		ObjectDemo obj2 = new ObjectDemo();
		obj2.SetFlag(new Integer(20));
		Integer falg2 = (Integer)obj2.getFalg();
		System.out.println("falg2:" + falg2);
		
		String falg3 = (String)obj2.getFalg();
		System.out.println("falg3:" + falg3);
		
	}

　　

然后String falg3 = (String)obj2.getFalg();异常

java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
	at com.example.fanxing.ObjectDemo.main(ObjectDemo.java:27)

　　

2、泛型的使用

public class GenericFlag<T> {
	
	private T  flag;

	public T getFlag() {
		return flag;
	}

	public void setFlag(T flag) {
		this.flag = flag;
	}
	
	public static void main(String[] args) {
		GenericFlag<Boolean> flag1 = new GenericFlag<Boolean>();
		flag1.setFlag(new Boolean(false));
		
		GenericFlag<Integer> flag2 = new GenericFlag<Integer>();
		flag2.setFlag(new Integer(20));
		
		Boolean f1 = flag1.getFlag();
		System.out.println("f1: " + f1);
		
		Integer f2 = flag2.getFlag();
		System.out.println("f2: " + f2);
	}
}

　　

3、两个泛型的实现方式

public class Generic<T1, T2> {

	private T1 flag1;
	
	private T2 flag2;

	public T1 getFlag1() {
		return flag1;
	}

	public void setFlag1(T1 flag1) {
		this.flag1 = flag1;
	}

	public T2 getFlag2() {
		return flag2;
	}

	public void setFlag2(T2 flag2) {
		this.flag2 = flag2;
	}
	
	public static void main(String[] args) {
		Generic<Integer, Boolean> flag = new Generic<Integer, Boolean>();
		
		flag.setFlag1(new Integer(20));
		flag.setFlag2(new Boolean(false));
		
		System.out.println(flag.getFlag1());
		System.out.println(flag.getFlag2());
	}
	
	
}

　　

4、数组类型的泛型

public class Generic2<T> {

	private T[] flagArray;

	public T[] getFlagArray() {
		return flagArray;
	}

	public void setFlagArray(T[] flagArray) {
		this.flagArray = flagArray;
	}
	
	
	public static void main(String[] args) {
		Generic2<String> flag = new Generic2<String>();
		String[] str1 = {"Nick","Tom","Larry"};
		
		String[] str2 = null;
		
		flag.setFlagArray(str1);
		
		str2 = flag.getFlagArray();
		
		for(int i = 0; i< str2.length ; i++){
			System.out.println(str2[i]);
		}
	}
}

　　

5、模拟ArrayList功能

public class SimpleCollection<T> {

	private T[] objArr;
	
	private int index = 0;
	
	
	public SimpleCollection() {
		objArr = (T[]) new Object[10];
	}
	
	public SimpleCollection(int capacity) {
		objArr = (T[]) new Object[capacity];
	}
	
	public void  add(T t) {
		objArr[index++] = t;
	}
	
	public int getLength() {
		return this.index;
	}
	
	public T get(int i){
		return objArr[i];
	}
	
	public static void main(String[] args) {
		SimpleCollection<Integer> s = new SimpleCollection<Integer>();
		for(int i = 0; i < 10; i++){
			s.add(new Integer(i));
		}
		
		for(int i = 0; i < 10; i++){
			Integer in = s.get(i);
			System.out.println(in);
		}
	}
	
}

　打印结果

0
1
2
3
4
5
6
7
8
9

　　

　6、泛型的泛型

/**
 * 泛型的泛型
 *
 */
public class WrapperFlag<T> {
	private GenericFlag3<T>  flag;

	public GenericFlag3<T> getFlag() {
		return flag;
	}

	public void setFlag(GenericFlag3<T> flag) {
		this.flag = flag;
	}
	
	public static void main(String[] args) {
		GenericFlag3<Integer> flag = new GenericFlag3<Integer>();
		flag.setFlag(100);
		
		WrapperFlag<Integer> wrapperFlag = new WrapperFlag<Integer>();
		wrapperFlag.setFlag(flag);
		
		GenericFlag3<Integer> g = wrapperFlag.getFlag();
		System.out.println(g.getFlag());
	}
	
}


class GenericFlag3<T>{
	private T flag;

	public T getFlag() {
		return flag;
	}

	public void setFlag(T flag) {
		this.flag = flag;
	}
	
}

　　

6、Set泛型

public class SetTest {
	
	public static void main(String[] args) {
		Set<String> set = new HashSet<String>();
		set.add("a1");
		set.add("a2");
		set.add("a3");
		
		for(Iterator<String> iterator = set.iterator(); iterator.hasNext();){
			String value = iterator.next();
			System.out.println(value);
		}
	}
	
}

　　

7、Set泛型，类型为对象Student

public class SetTest {
	
	public static void main(String[] args) {
		
		
		Set<Student> set2 = new HashSet<Student>();
		set2.add(new Student("tom", 20, "shanghai"));
		set2.add(new Student("Jack", 30, "ShangHai"));
		set2.add(new Student("Nick", 40, "YunNan"));
		
		for(Iterator<Student> iterator = set2.iterator(); iterator.hasNext();){
			Student student = iterator.next();
			String name = student.getName();
			int age = student.getAge();
			String address = student.getAddress();
			System.out.println(name + "," + age +"," + address);
		}
	}
	
}

class Student{
	private String name;
	private int age;
	private String address;

	public Student(String name, int age, String address) {
		this.name = name;
		this.age = age;
		this.address = address;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((address == null) ? 0 : address.hashCode());
		result = prime * result + age;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Student other = (Student) obj;
		if (address == null) {
			if (other.address != null)
				return false;
		} else if (!address.equals(other.address))
			return false;
		if (age != other.age)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}
	
	
}

　　

8、Map泛型

public class MapTest {

	public static void main(String[] args) {
		Map<String, String> map  = new HashMap<String, String>();
		map.put("a", "aa");
		map.put("b", "bb");
		map.put("c", "cc");
		map.put("d", "dd");
		
		//方式一
		Set<String> set = map.keySet();
		for(Iterator<String> iter = set.iterator(); iter.hasNext();){
			String key = iter.next();
			String value = map.get(key);
			System.out.println("key:" + key + " value:" + value);	
		}
		
		System.out.println("--------------------");
		
		//方式二
		Set<Map.Entry<String, String>> entriesSet = map.entrySet();
		for(Iterator<Map.Entry<String, String>> iter = entriesSet.iterator(); iter.hasNext();){
			Map.Entry<String, String> entry = iter.next();
			String key = entry.getKey();
			String value = entry.getValue();
			System.out.println("key:" + key + " value:" + value);	
			
		}
	}
}