# 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**主要用于對**自定義對象**的數組（或列表）進行排序。 實現`Comparable`接口的對象列表（和數組）可以由`Collections.sort`（和`Arrays.sort`）自動排序。在我們看到如何對自定義對象的對象進行排序之前，讓我們看看如何對已經實現 `Comparable`的數組元素和包裝器類進行排序。 ## 示例：對數組和包裝器類進行排序 ```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+", "); } } ``` **輸出：** ```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，您已經看到對實現`Comparable`接口的數組和對象列表進行排序是多么容易，您只需要調用`Collections.sort`（和`Arrays.sort`）。 **但是，如果要對自定義類的對象進行排序，則需要在自定義類中實現`Comparable`接口。** 該接口只有一個方法： ```java public abstract int compareTo(T obj) ``` 由于此方法是抽象的，因此如果實現`Comparable`接口，則必須在類中實現此方法。 讓我們舉個例子來更好地理解這個： ## 示例：通過實現`Comparable`接口對自定義對象進行排序 正如您所看到的，我在`Author`類中實現了`Comparable`接口，因為我想對這個類的對象進行排序。我已經在`compareTo()`方法中編寫了排序邏輯，您可以根據需求編寫邏輯。我想先按姓氏排序作者姓名，如果姓氏相同，則按名字排序。如果您只想按姓氏排序，那么`compareTo()`方法中的第一行就足夠了。 **`Author`類** ```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; } } ``` **排序類：`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); } } } ``` **輸出：** ```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：我們應該以這樣的方式編寫`compareTo()`方法：如果這個（我在這里指的是`this`關鍵字）小于傳遞的對象那么它應該返回負數，如果大于正數則為零，如果相等則返回 0。 你可能想知道為什么我沒有寫那個邏輯？因為名字和姓氏是字符串，所以我調用了字符串類的[`compareTo()`方法](https://beginnersbook.com/2013/12/java-string-compareto-method-example/)，它完全相同。 但是如果我們比較的東西是其他類型的東西，比如`int`那么你就可以編寫這樣的邏輯： 假設`Employee`類的對象是（`empId`，`empName`，`empAge`），我們想通過`empAge`對對象進行排序]。 ```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; } ```