44

我的数组不包含任何字符串。但它包含对象引用。每个对象引用都通过 toString 方法返回名称、id、作者和发布者。

public String toString() {
        return (name + "\n" + id + "\n" + author + "\n" + publisher + "\n");
}

现在我需要按名称对该对象数组进行排序。我知道如何排序，但我不知道如何从对象中提取名称并对它们进行排序。

4

11 回答 11

52

你可以尝试这样的事情：

List<Book> books = new ArrayList<Book>();

Collections.sort(books, new Comparator<Book>(){

  public int compare(Book o1, Book o2)
  {
     return o1.name.compareTo(o2.name);
  }
});
于 2013-09-19T13:43:07.003 回答
47

您有两种方法可以做到这一点，都使用Arrays实用程序类

  1. 实现一个比较器并将您的数组与比较器一起传递给将其作为第二个参数的排序方法。
  2. 在您的对象所在的类中实现Comparable接口，并将您的数组传递给仅采用一个参数的排序方法。

例子

class Book implements Comparable<Book> {
    public String name, id, author, publisher;
    public Book(String name, String id, String author, String publisher) {
        this.name = name;
        this.id = id;
        this.author = author;
        this.publisher = publisher;
    }
    public String toString() {
        return ("(" + name + ", " + id + ", " + author + ", " + publisher + ")");
    }
    @Override
    public int compareTo(Book o) {
        // usually toString should not be used,
        // instead one of the attributes or more in a comparator chain
        return toString().compareTo(o.toString());
    }
}

@Test
public void sortBooks() {
    Book[] books = {
            new Book("foo", "1", "author1", "pub1"),
            new Book("bar", "2", "author2", "pub2")
    };

    // 1. sort using Comparable
    Arrays.sort(books);
    System.out.println(Arrays.asList(books));

    // 2. sort using comparator: sort by id
    Arrays.sort(books, new Comparator<Book>() {
        @Override
        public int compare(Book o1, Book o2) {
            return o1.id.compareTo(o2.id);
        }
    });
    System.out.println(Arrays.asList(books));
}

输出

[(bar, 2, author2, pub2), (foo, 1, author1, pub1)]
[(foo, 1, author1, pub1), (bar, 2, author2, pub2)]
于 2013-09-19T13:46:21.647 回答
40

爪哇 8

使用lambda 表达式

Arrays.sort(myTypes, (a,b) -> a.name.compareTo(b.name));

测试.java

public class Test {

    public static void main(String[] args) {

        MyType[] myTypes = {
                new MyType("John", 2, "author1", "publisher1"),
                new MyType("Marry", 298, "author2", "publisher2"),
                new MyType("David", 3, "author3", "publisher3"),
        };

        System.out.println("--- before");
        System.out.println(Arrays.asList(myTypes));
        Arrays.sort(myTypes, (a, b) -> a.name.compareTo(b.name));
        System.out.println("--- after");
        System.out.println(Arrays.asList(myTypes));

    }

}

我的类型.java

public class MyType {

    public String name;
    public int id;
    public String author;
    public String publisher;

    public MyType(String name, int id, String author, String publisher) {
        this.name = name;
        this.id = id;
        this.author = author;
        this.publisher = publisher;
    }

    @Override
    public String toString() {
        return "MyType{" +
                "name=" + name + '\'' +
                ", id=" + id +
                ", author='" + author + '\'' +
                ", publisher='" + publisher + '\'' +
                '}' + System.getProperty("line.separator");
    }
}

输出：

--- before
[MyType{name=John', id=2, author='author1', publisher='publisher1'}
, MyType{name=Marry', id=298, author='author2', publisher='publisher2'}
, MyType{name=David', id=3, author='author3', publisher='publisher3'}
]
--- after
[MyType{name=David', id=3, author='author3', publisher='publisher3'}
, MyType{name=John', id=2, author='author1', publisher='publisher1'}
, MyType{name=Marry', id=298, author='author2', publisher='publisher2'}
]

使用方法引用

Arrays.sort(myTypes, MyType::compareThem);

compareThem必须在 MyType.java 中添加的位置：

public static int compareThem(MyType a, MyType b) {
    return a.name.compareTo(b.name);
}
于 2016-02-08T13:53:35.157 回答
12

Java 8 结构的更新

假设一个Book类具有name字段 getter，您可以通过传递一个使用 Java 8 构造指定Arrays.sort的附加方法来使用方法-比较器默认方法和方法引用。Comparator

Arrays.sort(bookArray, Comparator.comparing(Book::getName));

此外，可以使用thenComparing方法在多个字段上进行比较。

Arrays.sort(bookArray, Comparator.comparing(Book::getName)
      .thenComparing(Book::getAuthor))
      .thenComparingInt(Book::getId));
于 2019-01-04T06:41:40.927 回答
3

在Java 8中，您可以使用引用方法。

您可以将compare方法添加到您的Book课程中

class Book {
    public static int compare(Book a, Book b) {
        return a.name.compareTo(b.name);
    }
}

然后你可以这样做：

Arrays.sort(books, Book::compare);

这是完整的示例：

class Book {
    String name;
    String author;

    public Book(String name, String author) {
        this.name = name;
        this.author = author;
    }

    public static int compareBooks(Book a, Book b) {
        return a.name.compareTo(b.name);
    }

    @Override
    public String toString() {
        return "name : " + name + "\t" + "author : " + author;
    }

    public static void main(String[] args) {
        Book[] books = {
                new Book("Book 3", "Author 1"),
                new Book("Book 2", "Author 2"),
                new Book("Book 1", "Author 3"),
                new Book("Book 4", "Author 4")
        };
        Arrays.sort(books, Book::compareBooks);
        Arrays.asList(books).forEach(System.out::println);
    }
}
于 2018-01-24T09:26:52.420 回答
3 
[Employee(name=John, age=25, salary=3000.0, mobile=9922001), 
 Employee(name=Ace, age=22, salary=2000.0, mobile=5924001), 
 Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)]

public void whenComparing_thenSortedByName() {
    Comparator<Employee> employeeNameComparator
            = Comparator.comparing(Employee::getName);
    Arrays.sort(employees, employeeNameComparator);
    assertTrue(Arrays.equals(employees, sortedEmployeesByName));
}

结果

[Employee(name=Ace, age=22, salary=2000.0, mobile=5924001), 
 Employee(name=John, age=25, salary=3000.0, mobile=9922001), 
 Employee(name=Keith, age=35, salary=4000.0, mobile=3924401)]
于 2020-09-04T11:38:57.450 回答
2 
 Arrays.sort(yourList,new Comparator<YourObject>() {

    @Override
    public int compare(YourObject o1, YourObject o2) {
        return compare(o1.getYourColumn(), o2.getYourColumn());
    }
});
于 2019-05-14T06:09:50.187 回答
0

您可以在要比较其对象的类上实现“Comparable”接口。

并在其中实现“compareTo”方法。

在 ArrayList 中添加类的实例

然后“java.utils.Collections.sort()”方法将发挥必要的作用。

这是--->（https://deva-codes.herokuapp.com/CompareOnTwoKeys）一个工作示例，其中对象基于两个键首先按 id 然后按名称进行排序。

于 2018-07-15T11:42:04.657 回答
0

有时您想根据任意值对对象数组进行排序。由于 compareTo() 始终使用有关实例的相同信息，因此您可能希望使用不同的技术。一种方法是使用标准排序算法。假设您有一个书籍数组，并且您想根据它们的高度对它们进行排序，该高度存储为一个 int 并且可以通过方法 getHeight() 访问。以下是如何对数组中的书籍进行排序。（如果您不想更改原始数组，只需制作一个副本并对其进行排序。）

`int tallest; // the index of tallest book found thus far
 Book temp; // used in the swap
 for(int a = 0; a < booksArray.length - 1; a++) {
   tallest = a; // reset tallest to current index
   // start inner loop at next index
   for(int b = a + 1; b < booksArray.length; b++)
     // check if the book at this index is taller than the
     // tallest found thus far
     if(booksArray[b].getHeight() > booksArray[tallest].getHeight())
       tallest = b;
   // once inner loop is complete, swap the tallest book found with
   // the one at the current index of the outer loop
   temp = booksArray[a];
   booksArray[a] = booksArray[tallest];
   booksArray[tallest] = temp;
 }`

完成此代码后，Book 对象的数组将按高度降序排序——室内设计师的梦想！

于 2017-12-20T19:10:37.120 回答
-1 
public class Student implements Comparable<Student> {

    private int sid;
    private String sname;

    public Student(int sid, String sname) {
        super();
        this.sid = sid;
        this.sname = sname;
    }

    public int getSid() {
        return sid;
    }

    public void setSid(int sid) {
        this.sid = sid;
    }

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }

    @Override
    public String toString() {
        return "Student [sid=" + sid + ", sname=" + sname + "]";
    }

    public int compareTo(Student o) {
        if (this.getSname().compareTo(o.getSname()) > 1) {
            return toString().compareTo(o.getSname());
        } else if (this.getSname().compareTo(o.getSname()) < 1) {
            return toString().compareTo(o.getSname());
        }
        return 0;
    }

}
于 2018-07-12T07:41:12.327 回答
-2 
import java.util.Collections;
import java.util.List;
import java.util.ArrayList;

public class Test {
    public static void main(String[] args) {
        List<Student> str = new ArrayList<Student>();
        str.add(new Student(101, "aaa"));
        str.add(new Student(104, "bbb"));
        str.add(new Student(103, "ccc"));
        str.add(new Student(105, "ddd"));
        str.add(new Student(104, "eee"));
        str.add(new Student(102, "fff"));

        Collections.sort(str);
        for (Student student : str) {
            System.out.println(student);
        }
    }
}
于 2018-07-12T07:43:20.290 回答