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.
11 Answers
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);
}
});
You have two ways to do that, both use the Arrays utility class
- Implement a Comparator and pass your array along with the comparator to the sort method which take it as second parameter.
- 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)]
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);
}
