Java - How to sort by Comparable, difference from Comparator

javautils

Comparable is an interface, and if a class implements this interface, it can be used to sort a list, etc.

The return value of Comparables compareTo()` method can be negative, positive, or zero. If a negative number is returned, the order of the arguments is moved down.

public interface Comparable<T> {
    public int compareTo(T o);
}

Class that implements Comparable

Comparable as You can create a class that implements . Just override the compareTo() method and implement the code that compares two objects (this and its argument).

public class Text implements Comparable<Text> {
    private String mText;

    public Text(String text) {
        mText = text;
    }

    public String getText() {
        return mText;
    }

    @Override
    public int compareTo(@NotNull Text right) {
        return getText().length() - right.getText().length();
    }
}

sorting

If you pass a list of Comparable objects as an argument to Collections.sort(), sorting is performed. The criteria for sorting is the compareTo() method implemented above.

public void comparableExample1() {
    List<Text> texts = new ArrayList<>();
    texts.add(new Text("This code is free software"));
    texts.add(new Text("you can redistribute it"));
    texts.add(new Text("under the terms of the GNU General Public License version 2 only"));
    texts.add(new Text("This code is distributed in the hope that it will be useful"));
    texts.add(new Text("Please contact Oracle"));
    texts.add(new Text("500 Oracle Parkway, Redwood Shores, CA 94065 USA"));

    // before sorting
    for (Text text : texts) {
        System.out.println(text.getText());
    }

    // sorting by character length (ascending order)
    Collections.sort(texts);

    // out after sorting
    System.out.println();
    for (Text text : texts) {
        System.out.println(text.getText());
    }
}

result

This code is free software
you can redistribute it
under the terms of the GNU General Public License version 2 only
This code is distributed in the hope that it will be useful
Please contact Oracle
500 Oracle Parkway, Redwood Shores, CA 94065 USA

Please contact Oracle
you can redistribute it
This code is free software
500 Oracle Parkway, Redwood Shores, CA 94065 USA
This code is distributed in the hope that it will be useful
under the terms of the GNU General Public License version 2 only

Differences from Comparator

Comparator is a class that has rules for sorting. The return value of compare() can be negative, positive, or zero.

Since it is usually used only once and not used, it is made as an anonymous object as follows.

List<String> strings = new ArrayList<>();
strings.add("This code is free software");
strings.add("you can redistribute it");
strings.add("under the terms of the GNU General Public License version 2 only");
strings.add("This code is distributed in the hope that it will be useful");
strings.add("Please contact Oracle");
strings.add("500 Oracle Parkway, Redwood Shores, CA 94065 USA");

Collections.sort(strings, new Comparator<String>() {
    @Override
    public int compare(String s1, String s2) {
        return s1.length() - s2.length();
    }
});

result

Please contact Oracle
you can redistribute it
This code is free software
500 Oracle Parkway, Redwood Shores, CA 94065 USA
This code is distributed in the hope that it will be useful
under the terms of the GNU General Public License version 2 only

For more details, refer to Java - How to Sort with Comparator.

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