关于泛型-白红宇

关于泛型

阅读量：5923 次

发布时间：2019-06-19

本文共 12863 字，大约阅读时间需要 42 分钟。

泛型:

泛型是JDK1.5中一个最重要的特性,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ClassCastexceptions的可能.在JDK1.5中,你可以声明一个集合接收或者返回的对象的类型.

泛型之前:

类别定义时的逻辑完全一样,只是里面成员变量的类型不同.

如果需要多个相似的类,需要定义多个文件,不同的只是变量的类别,而逻辑是完全一样的.

IntergerFoo.java

1 public class IntegerFoo{ 2     private Integer foo; 3  4     public Integer getFoo(){ 5         return foo; 6     } 7  8     public void setFoo(Integer foo){ 9         this.foo = foo;10     }11 }

BooleanFoo.java

1 public class BooleanFoo{ 2     private Boolean foo; 3  4     public Boolean getFoo()    { 5         return foo; 6     } 7  8     public void setFoo(Boolean foo){ 9         this.foo = foo;10     }11 }

如果有很多类型的话想用一个类型来替代以上等的类型怎么办,只能用Object类.

ObjectFoo.java

1 public class ObjectFoo{ 2     private Object foo; 3  4     public Object getFoo(){ 5         return foo; 6     } 7  8     public void setFoo(Object foo){ 9         this.foo = foo;10     }11     12     public static void main(String[] args){13         ObjectFoo foo1 = new ObjectFoo();14         ObjectFoo foo2 = new ObjectFoo();15         16         foo1.setFoo(new Boolean(false));17         Boolean b = (Boolean)foo1.getFoo();18         19         foo2.setFoo(new Integer(10));20         Integer i = (Integer)foo2.getFoo();21         22         ObjectFoo foo3 = new ObjectFoo();23         foo3.setFoo(new Boolean(false));24         25         String str = (String)foo3.getFoo();26         //这个时候编译不会报错,但是运行的时候Object不能转换成String类型的,一定会出错27         //java.lang.ClassCastException: java.lang.Boolean cannot be caast to java.lang.String28     }29 }

定义泛型类别:

如果使用泛型,只要代码在编译时没有出现警告,就不会遇到运行时ClassCastException

所谓泛型就是变量的类型参数化(不是变量本身,是变量的类型).

GenericFoo.java

1 public class GenericFoo
     
      { 2     private T foo; 3      4     public T getFoo(){ 5         return foo; 6     } 7  8     public void setFoo(T foo){ 9         this.foo = foo;10     }11 12     public static void main(String[] args){13         GenericFoo
      
        foo1 = new GenericFoo
       
        ();14         GenericFoo
        
          foo2 = new GenericFoo
         
          ();15         16         foo1.setFoo(new Boolean(false));17         foo2.setFoo(new Integer(3));18         19         Boolean b = foo1.getFoo();20         Integer i = foo2.getFoo();21         22         System.out.println(b);//打印输出:false23         System.out.println(i);//打印输出:324         25         //foo1 = foo2;26         //报错Type mismatch: cannot convert from GenericFoo
          
            to GenericFoo
           
            27 28 GenericFoo a = new GenericFoo();29 a.setFoo("hello");30 31 String str = (String)a.getFoo();32 System.out.println(str);//输出:hello33 }34 }

Generic.java

1 public class Generic
     
      { 2     private T1 foo1; 3      4     private T2 foo2; 5  6     public T1 getFoo1(){ 7         return foo1; 8     } 9 10     public void setFoo1(T1 foo1){11         this.foo1 = foo1;12     }13 14     public T2 getFoo2(){15         return foo2;16     }17 18     public void setFoo2(T2 foo2){19         this.foo2 = foo2;20     }21     22     public static void main(String[] args){23         Generic
      
        foo = new Generic
       
        ();24         25         foo.setFoo1(new Integer(-20));26         foo.setFoo2(new Boolean(false));27         28         System.out.println(foo.getFoo1());29         System.out.println(foo.getFoo2());30     }31 }

打印输出:

-20

false

Generic2.java

1 public class Generic2
     
      { 2     private T[] fooArray; 3     //泛型数组 4  5     public T[] getFooArray(){ 6         return fooArray; 7     } 8  9     public void setFooArray(T[] fooArray){10         this.fooArray = fooArray;11     }12     13     public static void main(String[] args){14         Generic2
      
        foo = new Generic2
       
        ();15         16         String[] str1 = {"hello", "world", "welcome"};17         String[] str2 = null;18         19         foo.setFooArray(str1);20         21         str2 = foo.getFooArray();22         23         for(int i = 0; i < str2.length; i++){24             System.out.println(str2[i]);25         }26     }27 }

打印输出:

hello

world

welcome

SimpleCollection.java

1 public class SimpleCollection
     
      { 2     private T[] objArr; 3      4     private int index = 0; 5      6     public SimpleCollection(){ 7         objArr = (T[])new Object[10]; 8         //这个时候编译器会有一个警告: 9         //Type safety: Unchecked cast from Object[] to T[]10         //这个时候老师领着看的ArrayList的源码中的get()方法11         12         //objArr = new T[10];13         //报错:Cannot create a generic array of T14         //语法上不允许这样创建一个泛型数组.这个时候怎么办?15         //只能用Object类型.16         17     }18     19     public SimpleCollection(int capacity){20         objArr = (T[])new Object[capacity];21     }22     23     public void add(T t){24         objArr[index++] = t;25     }26     27     public int getLength(){28         return this.index;29     }30     31     public T get(int i){32         return objArr[i];33     }34     35     public static void main(String[] args){36         SimpleCollection
      
        c = new SimpleCollection
       
        ();37         38         for(int i = 0; i < 10; i++){39             c.add(new Integer(i));40         }41         42         for(int i = 0; i < 10; i++){43             Integer in = c.get(i);44             45             System.out.println(in);46         }47     }48 }

打印输出:1~9

ArrayListTest.java

1 public class ArrayListTest{ 2     public static void main(String[] args){ 3         List
     
       list = new ArrayList
      
       (); 4          5         list.add("a"); 6         list.add("b"); 7         list.add("c"); 8         list.add("d"); 9         //第一种遍历10         for(int i = 0; i < list.size(); i++){11             String value = list.get(i);12             System.out.println(value);13         }14         15         //第二种:使用迭代器16         for(Iterator
       
         iter = list.iterator(); iter.hasNext();){17             String value = iter.next();18             System.out.println(value);19         }20     }21 }

打印输出:两次a b c d

SetTest.java

1 package cn.shengsiyuan.jdk5;  2   3 import java.util.HashSet;  4 import java.util.Iterator;  5 import java.util.Set;  6   7 public class SetTest{  8     public static void main(String[] args){  9         Set
     
       set = new HashSet
      
       (); 10  11         set.add("aa"); 12         set.add("bb"); 13         set.add("cc"); 14  15         for (Iterator
       
         iter = set.iterator(); iter.hasNext();){ 16             String value = iter.next(); 17             System.out.println(value); 18         } 19          20         System.out.println("----------------"); 21          22         Set
        
          set2 = new HashSet
         
          (); 23          24         set2.add(new People("zhangsan", 20, "beijing")); 25         set2.add(new People("lisi", 30, "shanghai")); 26         set2.add(new People("wangwu", 40, "tianjin")); 27          28         for(Iterator
          
            iter = set2.iterator(); iter.hasNext();){ 29 People people = iter.next(); 30 31 String name = people.getName(); 32 int age = people.getAge(); 33 String address = people.getAddress(); 34 35 System.out.println(name + "," + age + "," + address); 36 } 37 } 38 } 39 40 class People{ 41 private String name; 42 43 private int age; 44 45 private String address; 46 47 public People(String name, int age, String address){ 48 this.name = name; 49 this.age = age; 50 this.address = address; 51 52 } 53 54 public int hashCode(){ 55 final int prime = 31; 56 int result = 1; 57 result = prime * result + ((address == null) ? 0 : address.hashCode()); 58 result = prime * result + age; 59 result = prime * result + ((name == null) ? 0 : name.hashCode()); 60 return result; 61 } 62 63 public boolean equals(Object obj){ 64 if (this == obj) 65 return true; 66 if (obj == null) 67 return false; 68 if (getClass() != obj.getClass()) 69 return false; 70 People other = (People) obj; 71 if (address == null){ 72 if (other.address != null) 73 return false; 74 } 75 else if (!address.equals(other.address)){ 76 return false; 77 } 78 if (age != other.age){ 79 return false; 80 } 81 if (name == null){ 82 if (other.name != null){ 83 return false; 84 } 85 } 86 else if (!name.equals(other.name)){ 87 return false; 88 } 89 return true; 90 } 91 92 public String getName(){ 93 return name; 94 } 95 96 public void setName(String name){ 97 this.name = name; 98 } 99 100 public int getAge(){101 return age;102 }103 104 public void setAge(int age){105 this.age = age;106 }107 108 public String getAddress(){109 return address;110 }111 112 public void setAddress(String address){113 this.address = address;114 }115 }

----------------

wangwu,40,tianjin

zhangsan,20,beijing

lisi,30,shanghai

MapTest.java

1 public class MapTest{ 2     public static void main(String[] args){ 3         Map
     
       map = new HashMap
      
       (); 4  5         map.put("a", "aa"); 6         map.put("b", "bb"); 7         map.put("c", "cc"); 8         map.put("d", "dd"); 9 10         Set
       
         set = map.keySet();11 12         for (Iterator
        
          iter = set.iterator(); iter.hasNext();){13             String key = iter.next();14             String value = map.get(key);15 16             System.out.println(key + ":" + value);17         }18 19         System.out.println("----------------------------------");20 21         Set
         
          
           > set2 = map.entrySet();22 23 for (Iterator
           
            
             > iter = set2.iterator(); iter24 .hasNext();){25 Map.Entry
             
               entry = iter.next();26 27 String key = entry.getKey();28 String value = entry.getValue();29 30 System.out.println(key + ":" + value);31 }32 }33 }

d:dd

b:bb

c:cc

a:aa

----------------------------------

d:dd

b:bb

c:cc

a:aa

限制泛型可用类型:

在定义泛型类别时,预设可以使用任何的类型来实例化泛型类型中的类型,但是如果想要限制使用泛型类别时,只能用某个特定类型或者是其子类才能实例化该类型时,可以再定义类型时,使用extends关键字指定这个类型必须是继承某个类,或者实现某个接口.

当没有指定泛型继承的类型或接口时,默认使用T extends Object,所以默认情况下任何类型都可以作为参数传入.

ListGenericFoo.java

1 public class ListGenericFoo
     
       { 2     // 虽然这个地方List也是一个接口,但是在泛型中仍然要用extends.不能用implements. 3     // 不管面是类还是接口都要用extends 4     private T[] fooArray; 5  6     public T[] getFooArray() { 7         return fooArray; 8     } 9 10     public void setFooArray(T[] fooArray) {11         this.fooArray = fooArray;12     }13 14     public static void main(String[] args) {15         ListGenericFoo
      
        foo1 = new ListGenericFoo
       
        ();16         ListGenericFoo
        
          foo2 = new ListGenericFoo
         
          ();17 18         LinkedList[] linkedList = new LinkedList[10];19 20         foo1.setFooArray(linkedList);21 22         ArrayList[] arrayList = new ArrayList[10];23 24         foo2.setFooArray(arrayList);25 26         // ListGenericFoo
          
            foo3 = new ListGenericFoo
           
            ();27 // 报错,HashMap和List没有关系.28 // Bound mismatch: The type HashMap is not a valid substitute for29 // the bounded parameter 
            
              of the type ListGenericFoo
             
              30 }31 }

类型通配声明:

现在有这么一个需求,你希望哟u一个参考名称foo可以接受所有下面的实例:

foo = new Generic Foo
     
      ();foo = new Generic Foo
      
       ();

简单的说,实例化类型持有者时,它必须是实现List的类别或其子类别,要定义这样一个名称,你可以使用"?"通配字符,并使用extends关键字限定类型持有者的形态.

GenericFoo
      foo = null;foo = new GenericFoo
     
      ();foo = new GenericFoo
      
       ();

GenericTest.java

1 public class GenericTest
     
      { 2     private T foo; 3  4     public T getFoo(){ 5         return foo; 6     } 7  8     public void setFoo(T foo){ 9         this.foo = foo;10     }11     12     public static void main(String[] args){13         GenericTest
       ge = null;14         15         ge = new GenericTest
      
       ();16         ge = new GenericTest
       
        ();17         18         //ge = new GenericTest
        
         ();19         20         GenericTest
          ge2 = null;21         //这种super的非常少用,传递的值的类型必须是List的父类....22         ge2 = new GenericTest

使用<?>或是<? extends SomeClass>的声明方式，意味著您只能通过该名称來取得所参考实例的信息，或者是移除某些信息，但不能增加它的信息，因为只知道当中放置的是SomeClass的子类，但不确定是什么类的实例，编译器不让您加入信息，理由是，如果可以加入信息的話，那么您就得記得取回的实例是什么类型，然后转换为原來的类型方可进行操作，这就失去了使用泛型的意义。

转载地址：http://vqxvx.baihongyu.com/

你可能感兴趣的文章

jmeter高级用法例子，如何扩展自定义函数