Java中对象的比较

有两种比较方法，一种是对象实现Comparable接口并覆写compareTo方法，另一种是定义实现Comparator接口的类。

如果只是判断是否相等，就覆写equals方法

1) Employee对象实现Comparable接口

public class Employee implements Comparable<Employee>{/**员工姓名*/private String name;/**员工性别*/private String sex;/**员工年龄*/private int age;public Employee(String name,String sex,int age){//不再有默认空参构造啦this.name = name;this.sex = sex;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic int hashCode() {return super.hashCode();}@Overridepublic boolean equals(Object obj) {//只比较员工姓名Employee e = (Employee) obj;return this.name.equals(e.name);//竟然可以在这直接调用私有属性}@Overridepublic String toString() {//打印，获取信息return this.name+"-"+this.sex+"-"+this.age;}@Overridepublic int compareTo(Employee o) {//用年龄来比较if(this.age == o.getAge())return 0;else if(this.age > o.getAge())return 1;elsereturn -1;}}

2) 或者使用Comparator方式

public class EmployeeComparator implements Comparator<Employee> {@Overridepublic int compare(Employee o1, Employee o2) {//充分利用三目运算符int result = o1.getAge() == o2.getAge()?0:(o1.getAge()>o2.getAge()?1:-1);return result;}}

3) 下面是使用到比较的例子

List<Employee> list = new ArrayList<Employee>();list.add(new Employee("总监", "男", 30));list.add(new Employee("项目经理", "男", 28));list.add(new Employee("攻城狮", "男", 27));list.add(new Employee("设计师", "女", 22));list.add(new Employee("总经理", "男", 35));list.add(new Employee("董事长", "男", 40));//Collections.sort(list);//使用ComparableCollections.sort(list, new EmployeeComparator());//使用ComparatorSystem.out.println(list);//调toStringSet<String> mSet = new TreeSet<String>();//排序，所插对象需实现Comparable接口mSet.add("zhang");mSet.add("wang");mSet.add("li");mSet.add("zhao");mSet.add("zhang");System.out.println(mSet);Map<Integer, String> map = new TreeMap<Integer, String>();//排序，所插对象需实现Comparable接口map.put(4, "washing machine");map.put(1, "television");map.put(2, "watch");map.put(3, "refrigerator");map.put(5, "computer");System.out.println(map);

4)Comparable和Comparator的差别

Comparable需要比较的类本身实现此接口，Comparator不需要。

String是final类，本身实现了Comparable接口，但不能被继承。即如果我们对其compareTo方法不满意，又不能覆写它。只能使用Comparator了。

5)一些实用的方法

Collections.addAll(Collection<String>, String[])。将数组转化为Collection，或理解为将数组中的数据全部添加到Collection

Arrays.asList(String[])。将数组转化为List，List.toString方便查看所有数据

Collection.toArray(). 将Collection转化为数组，数组遍历效率较高

Collections.sort(List)及Collections.sort(List,Comparator). 对List排序

Collections.shuffle(List)及Collections.shuffle(List,Random). 将List顺序打乱

6)使用Comparator对Map和Set排序，即使用new TreeMap(Comparator)和new TreeSet(Comparator)

public class StringComparator implements Comparator<String> {@Overridepublic int compare(String lhs, String rhs) {String[] str1 = lhs.split("\\.");String[] str2 = rhs.split("\\.");for(int i=0;i<str1.length-1;i++){int int1 = Integer.parseInt(str1[i]);int int2 = Integer.parseInt(str2[i]);if(int1>int2){return 1;}else if(int1<int2){return -1;}}return 0;}}

Map<String, Set<String>> map = new TreeMap<String, Set<String>>(new StringComparator());String[] data = {"1.1.html","4.77.html","10.55.html","2.33.html","1.html","4.html","1.5.html","10.html","2.html"};for(int i=0;i<data.length;i++){if(data[i].indexOf('.')!=data[i].lastIndexOf('.')){String key = data[i].substring(0, data[i].indexOf('.'))+data[i].substring(data[i].lastIndexOf('.'));if(map.get(key)==null){map.put(key, new TreeSet<String>(new StringComparator()));}map.get(key).add(data[i]);}}

TreeMap实现Navigable接口，有几个实用的方法：lowerKey, higherKey, floorKey, ceilingKey.

TreeSet实现SortedSet接口，有几个实用方法：first(), last().