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。
【问题讨论】：
Comparator 并将其用于排序。
Comparator中拆分字符串并使用第一个元素作为名称。
标签： java arrays string sorting object
你可以试试这样的：
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);
}
});
【讨论】：
您有两种方法可以做到这一点，都使用 Arrays 实用程序类
示例
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)]
【讨论】：
Arrays.sort(myTypes, (a,b) -> a.name.compareTo(b.name));
Test.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));
}
}
MyType.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'}
]
Arrays.sort(myTypes, MyType::compareThem);
compareThem 必须在 MyType.java 中添加：
public static int compareThem(MyType a, MyType b) {
return a.name.compareTo(b.name);
}
【讨论】：
假设Book 类具有name 字段getter，您可以通过传递使用Java 8 构造指定的额外Comparator - Comparator default method 和method references 来使用Arrays.sort 方法。
Arrays.sort(bookArray, Comparator.comparing(Book::getName));
此外，还可以使用thenComparing 方法对多个字段进行比较。
Arrays.sort(bookArray, Comparator.comparing(Book::getName)
.thenComparing(Book::getAuthor))
.thenComparingInt(Book::getId));
【讨论】：
[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)]
【讨论】：
使用 Java 8，您可以使用引用方法。
您可以将 compare 方法添加到您的 Book 类中
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);
这里是完整的例子：
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);
}
}
【讨论】：
Arrays.sort(yourList,new Comparator<YourObject>() {
@Override
public int compare(YourObjecto1, YourObjecto2) {
return compare(o1.getYourColumn(), o2.getYourColumn());
}
});
【讨论】：
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，只需复制并排序即可。）
`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！
【讨论】：
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()”方法将发挥必要的魔力。
这是--->(https://deva-codes.herokuapp.com/CompareOnTwoKeys) 一个工作示例，其中对象基于两个键排序，首先按 id，然后按名称。
【讨论】：
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;
}
}
【讨论】：
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);
}
}
}
【讨论】：