# Java 中的`Comparable`接口 > 原文： [https://beginnersbook.com/2017/08/comparable-interface-in-java-with-example/](https://beginnersbook.com/2017/08/comparable-interface-in-java-with-example/) **`Comparable`接口**主要用于对**自定义对象**的数组（或列表）进行排序。 实现`Comparable`接口的对象列表（和数组）可以由`Collections.sort`（和`Arrays.sort`）自动排序。在我们看到如何对自定义对象的对象进行排序之前，让我们看看如何对已经实现 `Comparable`的数组元素和包装器类进行排序。 ## 示例：对数组和包装器类进行排序 ```java import java.util.ArrayList; import java.util.Arrays; import java.util.Collections; import java.util.List; public class Demo { public static void main(String[] args) { /* * Integer class implements Comparable * Interface so we can use the sort method */ int[] arr = {11,55,22,0,89}; Arrays.sort(arr); System.out.print("Sorted Int Array: "); System.out.println(Arrays.toString(arr)); /* * String class implements Comparable * Interface so we can use the sort method */ System.out.print("Sorted String Array: "); String[] names = {"Steve", "Ajeet", "Kyle"}; Arrays.sort(names); System.out.println(Arrays.toString(names)); /* * String class implements Comparable * Interface so we can use the sort method */ System.out.print("Sorted List: "); List fruits = new ArrayList(); fruits.add("Orange"); fruits.add("Banana"); fruits.add("Apple"); fruits.add("Guava"); fruits.add("Grapes"); Collections.sort(fruits); for(String s: fruits) System.out.print(s+", "); } } ``` **输出：** ```java Sorted Int Array: [0, 11, 22, 55, 89] Sorted String Array: [Ajeet, Kyle, Steve] Sorted List: Apple, Banana, Grapes, Guava, Orange, ``` 在上面的示例中，您已经看到对实现`Comparable`接口的数组和对象列表进行排序是多么容易，您只需要调用`Collections.sort`（和`Arrays.sort`）。 **但是，如果要对自定义类的对象进行排序，则需要在自定义类中实现`Comparable`接口。** 该接口只有一个方法： ```java public abstract int compareTo(T obj) ``` 由于此方法是抽象的，因此如果实现`Comparable`接口，则必须在类中实现此方法。 让我们举个例子来更好地理解这个： ## 示例：通过实现`Comparable`接口对自定义对象进行排序 正如您所看到的，我在`Author`类中实现了`Comparable`接口，因为我想对这个类的对象进行排序。我已经在`compareTo()`方法中编写了排序逻辑，您可以根据需求编写逻辑。我想先按姓氏排序作者姓名，如果姓氏相同，则按名字排序。如果您只想按姓氏排序，那么`compareTo()`方法中的第一行就足够了。 **`Author`类** ```java public class Author implements Comparable<Author> { String firstName; String lastName; String bookName; Author(String first, String last, String book){ this.firstName = first; this.lastName = last; this.bookName = book; } @Override /* * This is where we write the logic to sort. This method sort * automatically by the first name in case that the last name is * the same. */ public int compareTo(Author au){ /* * Sorting by last name. compareTo should return < 0 if this(keyword) * is supposed to be less than au, > 0 if this is supposed to be * greater than object au and 0 if they are supposed to be equal. */ int last = this.lastName.compareTo(au.lastName); //Sorting by first name if last name is same d return last == 0 ? this.firstName.compareTo(au.firstName) : last; } } ``` **排序类：`SortAuthByNames`** ```java import java.util.ArrayList; import java.util.Collections; public class SortAuthByNames{ public static void main(String args[]){ // List of objects of Author class ArrayList<Author> al=new ArrayList<Author>(); al.add(new Author("Henry","Miller", "Tropic of Cancer")); al.add(new Author("Nalo","Hopkinson", "Brown Girl in the Ring")); al.add(new Author("Frank","Miller", "300")); al.add(new Author("Deborah","Hopkinson", "Sky Boys")); al.add(new Author("George R. R.","Martin", "Song of Ice and Fire")); /* * Sorting the list using Collections.sort() method, we * can use this method because we have implemented the * Comparable interface in our user defined class Author */ Collections.sort(al); for(Author str:al){ System.out.println(str.firstName+" "+ str.lastName+" "+"Book: "+str.bookName); } } } ``` **输出：** ```java Deborah Hopkinson Book: Sky Boys Nalo Hopkinson Book: Brown Girl in the Ring George R. R. Martin Book: A Song of Ice and Fire Frank Miller Book: 300 Henry Miller Book: Tropic of Cancer ``` > 注意：我们应该以这样的方式编写`compareTo()`方法：如果这个（我在这里指的是`this`关键字）小于传递的对象那么它应该返回负数，如果大于正数则为零，如果相等则返回 0。 你可能想知道为什么我没有写那个逻辑？因为名字和姓氏是字符串，所以我调用了字符串类的[`compareTo()`方法](https://beginnersbook.com/2013/12/java-string-compareto-method-example/)，它完全相同。 但是如果我们比较的东西是其他类型的东西，比如`int`那么你就可以编写这样的逻辑： 假设`Employee`类的对象是（`empId`，`empName`，`empAge`），我们想通过`empAge`对对象进行排序]。 ```java public int compareTo(Employee e){ if(this.empAge==e.empAge) return 0; else if(this.empAge>e.empAge) return 1; else return -1; } ``` **或** ```java public int compareTo(Employee e){ return this.empAge > e.empAge ? 1 : this.empAge < e.empAge ? -1 : 0; } ```