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一、概述：
- Collections是一个操作Set List Map的工具类
- 提供了一系列静态的方法对集合元素进行排序、查询和修改等操作，
- 还提供了对计划对象设置不可变、对计划对象实现同步控制等方法
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二、Collections常用方法
1、排序方法：（均为Static）
- reverse(List),反转List中元素的顺序；
- shuffle(List):对List重的元素随机排序
- sort(List) 根据元素的自然顺序对指定的List集合元素升序排序；
- sort(List,Comparator):根据指定的Comparator产生的顺序对List集合元素进行排序
- Swap(List,int i,int j)将指定List集合中的i处元素和j处元素进行交换；
2、取最值方法：
Object max(Collection):根据元素的自然顺、返回给定集合中最大元素；
Object max(Collection,Comparator):根据Comparator指定的顺序，返回给定集合中最大的元素
Object min(Collection)
Object min(Collection,Comparator):
3、其他方法
- int frequnency(Collection,Object),返回集合中元素出现的次数；
- void copy(List dest,List src),将src中的内容复制到dest中
- boolean replaceAll(List list,Object oldVal,Object newVal)使用新值，替换老的值
- Collections 类中提供了多个synchronizedXxx()方法；
该方法可以使得指定集合包装成线程同步的集合，从而可以解决多线程并发访问集合是的线程安全问题
4、测试方法代码：
1 @Test 2 public void test() { 3 ArrayList arrayList = new ArrayList(); 4 arrayList.add(123); 5 arrayList.add(456); 6 arrayList.add("AA"); 7 arrayList.add(new Person("Tom",1)); 8 arrayList.add(456); 9 System.out.println(arrayList.toString()); 10 11 Collections.reverse(arrayList); 12 System.out.println("Collections的 reverse方法"+arrayList); 13 14 15 } 16 @Test 17 public void test1() { 18 ArrayList arrayList = new ArrayList(); 19 arrayList.add(123); 20 arrayList.add(456); 21 arrayList.add("AA"); 22 arrayList.add(new Person("Tom",1)); 23 arrayList.add(456); 24 System.out.println(arrayList.toString()); 25 26 Collections.shuffle(arrayList); 27 System.out.println("Collections的 shuffle方法"+arrayList); 28 29 30 } 31 32 @Test 33 public void test2() { 34 ArrayList arrayList = new ArrayList(); 35 arrayList.add(new Person("Tom",12)); 36 arrayList.add(new Person("joy",13)); 37 arrayList.add(new Person("jerry",15)); 38 arrayList.add(new Person("lucy",1)); 39 System.out.println(arrayList.toString()); 40 41 Collections.sort(arrayList); 42 System.out.println("Collections的 sort的自然排序方法"+arrayList); 43 44 Collections.sort(arrayList,new Comparator<Person>() { 45 46 @Override 47 public int compare(Person o1, Person o2) { 48 if(o1.getAge()-o2.getAge()>1) { 49 return 1; 50 } 51 return -1; 52 } 53 }); 54 System.out.println("Collections的 sort和 comparator方法"+arrayList); 55 56 57 } 58 59 @Test 60 public void test3() { 61 ArrayList arrayList = new ArrayList(); 62 arrayList.add(new Person("Tom",12)); 63 arrayList.add(new Person("joy",13)); 64 arrayList.add(new Person("jerry",15)); 65 arrayList.add(new Person("lucy",1)); 66 System.out.println(arrayList.toString()); 67 Collections.swap(arrayList, 0, 1); 68 System.out.println("Collections的 swap方法"+arrayList); 69 } 70 71 @Test 72 public void test4() { 73 ArrayList arrayList = new ArrayList(); 74 arrayList.add(new Person("Tom",12)); 75 arrayList.add(new Person("joy",13)); 76 arrayList.add(new Person("jerry",15)); 77 arrayList.add(new Person("lucy",1)); 78 System.out.println(arrayList.toString()); 79 Person person = Collections.max(arrayList,new Comparator<Person>() { 80 81 @Override 82 public int compare(Person o1, Person o2) { 83 if(o1.getAge()-o2.getAge()>1) { 84 return 1; 85 } 86 return -1; } 87 }); 88 System.out.println("Collections的 max方法"+person); 89 } 90 91 @Test 92 public void test5() { 93 ArrayList arrayList = new ArrayList(); 94 arrayList.add(new Person("Tom",12)); 95 arrayList.add(new Person("joy",13)); 96 arrayList.add(new Person("jerry",15)); 97 arrayList.add(new Person("lucy",1)); 98 arrayList.add(new Person("lucy",1)); 99 System.out.println(arrayList.toString()); 100 int times = Collections.frequency(arrayList, new Person("lucy",1)); 101 System.out.println("Collections的 frequency方法"+times); 102 } 103 104 @Test 105 public void test6() { 106 ArrayList arrayList = new ArrayList(); 107 arrayList.add(new Person("Tom",12)); 108 arrayList.add(new Person("joy",13)); 109 arrayList.add(new Person("jerry",15)); 110 arrayList.add(new Person("lucy",1)); 111 arrayList.add(new Person("lucy",1)); 112 113 //错误 114 // ArrayList arrayList1 = new ArrayList(); 115 // Collections.copy(arrayList1, arrayList);; 116 List arrayList1 = Arrays.asList(new Object[arrayList.size()]); 117 Collections.copy(arrayList1, arrayList);; 118 System.out.println("Collections的 copy方法"+arrayList1); 119 } 120 121 @Test 122 public void test7() { 123 ArrayList arrayList = new ArrayList(); 124 arrayList.add(new Person("Tom",12)); 125 arrayList.add(new Person("joy",13)); 126 arrayList.add(new Person("jerry",15)); 127 arrayList.add(new Person("lucy",1)); 128 arrayList.add(new Person("lucy",1)); 129 130 Collections.replaceAll(arrayList, new Person("lucy",1), new Person("lucy",3)); 131 System.out.println("Collections的 replaceAll方法"+arrayList); 132 } 133 134 @Test 135 public void test8() { 136 ArrayList arrayList = new ArrayList(); 137 arrayList.add(new Person("Tom",12)); 138 arrayList.add(new Person("joy",13)); 139 arrayList.add(new Person("jerry",15)); 140 arrayList.add(new Person("lucy",1)); 141 arrayList.add(new Person("lucy",1)); 142 143 //返回线程安全的 list 144 List list = Collections.synchronizedList(arrayList); 145 System.out.println("Collections的 synchronizedList方法"+list); 146 }
