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在用 java 编程时，经常会有这样的需求：把 list 中的某一类的对象，按照某个对象属性对所有对象在内存中进行排序，那么我们改如何实现呢？ 答案是，实现 java 提供的 Comparable 接口。
我们模拟一个需求：按员工的工资从小到大排序员工对象。
我们首先定义个 Employee 类，有个3个实例属性: 员工 id（empId），名称（eName），薪资（sal）, 此类实现了 Comparable 接口。
[java]
public class Employee implements Comparable {
int empId;
String eName;
double sal;
static int i;
public Employee() {
empId = i++;
eName = "don't know";
sal = 0.0;
}
public Employee(String eName, double sal) {
this.empId = i++;
this.eName = eName;
this.sal = sal;
}
@Override
public String toString() {
return "empId " + empId + "\n" + "eName " + eName + "\n" + "sal " + sal;
}
@Override
public int compareTo(Object o) {
if (this.sal == ((Employee) o).sal) return 0;
else if (this.sal > ((Employee) o).sal) return 1;
else return -1;
}
}
public class Employee implements Comparable {
int empId;
String eName;
double sal;
static int i;
public Employee() {
empId = i++;
eName = "don't know";
sal = 0.0;
}
public Employee(String eName, double sal) {
this.empId = i++;
this.eName = eName;
this.sal = sal;
}
@Override
public String toString() {
return "empId " + empId + "\n" + "eName " + eName + "\n" + "sal " + sal;
}
@Override
public int compareTo(Object o) {
if (this.sal == ((Employee) o).sal) return 0;
else if (this.sal > ((Employee) o).sal) return 1;
else return -1;
}
}
我在定义个 ComparableDemo 类
[java]
public class ComparableDemo {
public static void main(String[] args) {
List list = new ArrayList();
list.add(new Employee("Tom",40000.00));
list.add(new Employee("Harry",20000.00));
list.add(new Employee("Maggle",50000.00));
list.add(new Employee("Chris",70000.00));
Collections.sort(list);
Iterator iterator = list.iterator();
while(iterator.hasNext()){
Object element = iterator.next();
System.out.println(element.toString()+"\n");
}
}
}
public class ComparableDemo {
public static void main(String[] args) {
List list = new ArrayList();
list.add(new Employee("Tom",40000.00));
list.add(new Employee("Harry",20000.00));
list.add(new Employee("Maggle",50000.00));
list.add(new Employee("Chris",70000.00));
Collections.sort(list);
Iterator iterator = list.iterator();
while(iterator.hasNext()){
Object element = iterator.next();
System.out.println(element.toString()+"\n");
}
}
}
执行 ComparableDemo main 方法输出如下：
empId 1
eName Harry
sal 20000.0
empId 0
eName Tom
sal 40000.0
empId 2
eName Maggle
sal 50000.0
empId 3
eName Chris
sal 70000.0
实现注意点：
类 Employee 实现了 Comparable 接口，代表此类的对象是可排序的
类 Employee 实现了 Comparable 接口里的 compareTo 方法，此方法里实现的排序的逻辑
类 ComparableDemo 调用 Collections.sort(list); 对 list 的 Employee 的对象按照排序逻辑进行排序
既然有了 Comparable 接口，为什么还需要 Comparator 接口呢？
在这，我提出一个假设：如果我们的 Employee 类按照不同的实例属性排序呢？上面的例子是按照薪资从小到大排序，如果需求变化了，Employee 除了支持按 薪资(sal) 排序，还要支持按照名称(eName) 排序。现在就需要 Comparator 接口来实现了。
我们新建 NameComparator 类，实现 java.util.Comparator 接口，如下：
[java]
public class NameComparator implements Comparator {
@Override
public int compare(Object o1, Object o2) {
Employee e1 = (Employee)o1;
Employee e2 = (Employee)o2;
return e1.eName.compareTo(e2.eName);
}
}
public class NameComparator implements Comparator {
@Override
public int compare(Object o1, Object o2) {
Employee e1 = (Employee)o1;
Employee e2 = (Employee)o2;
return e1.eName.compareTo(e2.eName);
}
}
我们修改 ComparableDemo 类的代码如下：
[java]
public class ComparableDemo {
public static void main(String[] args) {
List list = new ArrayList();
list.add(new Employee("Tom", 40000.00));
list.add(new Employee("Harry", 20000.00));
list.add(new Employee("Maggle", 50000.00));
list.add(new Employee("Chris", 70000.00));
<SPAN style="COLOR: #ff0000">//Collections.sort(list);
Collections.sort(list, new NameComparator());</SPAN>
Iterator iterator = list.iterator();
while (iterator.hasNext()) {
Object element = iterator.next();
System.out.println(element.toString() + "\n");
}
}
}
public class ComparableDemo {
public static void main(String[] args) {
List list = new ArrayList();
list.add(new Employee("Tom", 40000.00));
list.add(new Employee("Harry", 20000.00));
list.add(new Employee("Maggle", 50000.00));
list.add(new Employee("Chris", 70000.00));
//Collections.sort(list);
Collections.sort(list, new NameComparator());
Iterator iterator = list.iterator();
while (iterator.hasNext()) {
Object element = iterator.next();
System.out.println(element.toString() + "\n");
}
}
}运行 ComparableDemo ，结果如下：
empId 3
eName Chris
sal 70000.0
empId 1
eName Harry
sal 20000.0
empId 2
eName Maggle
sal 50000.0
empId 0
eName Tom
sal 40000.0
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