How to sort an array of objects in Java?

Question

My array does not contain any string. But its contains object references. Every object reference returns name, id, author and publisher by toString method.

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

Now I need to sort that array of objects by the name. I know how to sort, but I do not know how to extract the name from the objects and sort them.

Answer 1

Java 8

---

Using lambda expressions

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");
    }
}

Output:

--- 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'}
]

---

Using method references

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

where compareThem has to be added in MyType.java:

public static int compareThem(MyType a, MyType b) {
    return a.name.compareTo(b.name);
}

Answer 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, "ffffd"));

   str.add(new Student(104, "eee"));

   str.add(new Student(102, "fff"));




   Collections.sort(str);
    for(Student student : str) {

        System.out.println(student);
    }

}
}

Answer 3

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;
    }

}

Answer 4

You can try something like this:

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);
  }
});

Answer 5

You can implement the "Comparable" interface on a class whose objects you want to compare.

And also implement the "compareTo" method in that.

Add the instances of the class in an ArrayList

Then the "java.utils.Collections.sort()" method will do the necessary magic.

Here's--->(https://deva-codes.herokuapp.com/CompareOnTwoKeys) a working example where objects are sorted based on two keys first by the id and then by name.

Answer 6

You have two ways to do that, both use the Arrays utility class

1. Implement a Comparator and pass your array along with the comparator to the sort method which take it as second parameter.
2. Implement the Comparable interface in the class your objects are from and pass your array to the sort method which takes only one parameter.

Example

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));
}

Output

[(bar, 2, author2, pub2), (foo, 1, author1, pub1)]
[(foo, 1, author1, pub1), (bar, 2, author2, pub2)]