Java 8 Lambda: Vergleichsbeispiel

java-lambda-expression

In diesem Beispiel zeigen wir Ihnen, wie Sie mit dem Java 8 Lambda-AusdruckComparator schreiben, um eine Liste zu sortieren.

1. Beispiel für ClassicComparator.

    Comparator byName = new Comparator() {
        @Override
        public int compare(Developer o1, Developer o2) {
            return o1.getName().compareTo(o2.getName());
        }
    };

2. Äquivalent zum Lambda-Ausdruck.

    Comparator byName =
        (Developer o1, Developer o2)->o1.getName().compareTo(o2.getName());

1. Sortieren ohne Lambda

Beispiel zum Vergleichen derDeveloper-Objekte anhand ihres Alters. Normalerweise verwenden SieCollections.sort und übergeben eine anonymeComparator-Klasse wie folgt:

TestSorting.java

package com.example.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class TestSorting {

    public static void main(String[] args) {

        List listDevs = getDevelopers();

        System.out.println("Before Sort");
        for (Developer developer : listDevs) {
            System.out.println(developer);
        }

        //sort by age
        Collections.sort(listDevs, new Comparator() {
            @Override
            public int compare(Developer o1, Developer o2) {
                return o1.getAge() - o2.getAge();
            }
        });

        System.out.println("After Sort");
        for (Developer developer : listDevs) {
            System.out.println(developer);
        }

    }

    private static List getDevelopers() {

        List result = new ArrayList();

        result.add(new Developer("example", new BigDecimal("70000"), 33));
        result.add(new Developer("alvin", new BigDecimal("80000"), 20));
        result.add(new Developer("jason", new BigDecimal("100000"), 10));
        result.add(new Developer("iris", new BigDecimal("170000"), 55));

        return result;

    }

}

Ausgabe

Before Sort
Developer [name=example, salary=70000, age=33]
Developer [name=alvin, salary=80000, age=20]
Developer [name=jason, salary=100000, age=10]
Developer [name=iris, salary=170000, age=55]

After Sort
Developer [name=jason, salary=100000, age=10]
Developer [name=alvin, salary=80000, age=20]
Developer [name=example, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]

Wenn die Sortieranforderung geändert wird, übergeben Sie einfach eine andere neue anonymeComparator-Klasse:

    //sort by age
    Collections.sort(listDevs, new Comparator() {
        @Override
        public int compare(Developer o1, Developer o2) {
            return o1.getAge() - o2.getAge();
        }
    });

    //sort by name
    Collections.sort(listDevs, new Comparator() {
        @Override
        public int compare(Developer o1, Developer o2) {
            return o1.getName().compareTo(o2.getName());
        }
    });

    //sort by salary
    Collections.sort(listDevs, new Comparator() {
        @Override
        public int compare(Developer o1, Developer o2) {
            return o1.getSalary().compareTo(o2.getSalary());
        }
    });

Es funktioniert, aber finden Sie es etwas seltsam, eine Klasse zu erstellen, nur weil Sie eine einzelne Codezeile ändern möchten?

2. Sortieren mit Lambda

In Java 8 unterstützt dieList-Schnittstelle diesort-Methode direkt, sodassCollections.sort nicht mehr verwendet werden muss.

        //List.sort() since Java 8
    listDevs.sort(new Comparator() {
        @Override
        public int compare(Developer o1, Developer o2) {
            return o2.getAge() - o1.getAge();
        }
    });

Beispiel für einen Lambda-Ausdruck:

TestSorting.java

package com.example.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

public class TestSorting {

    public static void main(String[] args) {

        List listDevs = getDevelopers();

        System.out.println("Before Sort");
        for (Developer developer : listDevs) {
            System.out.println(developer);
        }

        System.out.println("After Sort");

        //lambda here!
        listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());

        //java 8 only, lambda also, to print the List
        listDevs.forEach((developer)->System.out.println(developer));
    }

    private static List getDevelopers() {

        List result = new ArrayList();

        result.add(new Developer("example", new BigDecimal("70000"), 33));
        result.add(new Developer("alvin", new BigDecimal("80000"), 20));
        result.add(new Developer("jason", new BigDecimal("100000"), 10));
        result.add(new Developer("iris", new BigDecimal("170000"), 55));

        return result;

    }

}

Ausgabe

Before Sort
Developer [name=example, salary=70000, age=33]
Developer [name=alvin, salary=80000, age=20]
Developer [name=jason, salary=100000, age=10]
Developer [name=iris, salary=170000, age=55]

After Sort
Developer [name=jason, salary=100000, age=10]
Developer [name=alvin, salary=80000, age=20]
Developer [name=example, salary=70000, age=33]
Developer [name=iris, salary=170000, age=55]

3. Weitere Lambda-Beispiele

3.1 Sort By age

    //sort by age
    Collections.sort(listDevs, new Comparator() {
        @Override
        public int compare(Developer o1, Developer o2) {
            return o1.getAge() - o2.getAge();
        }
    });

    //lambda
    listDevs.sort((Developer o1, Developer o2)->o1.getAge()-o2.getAge());

    //lambda, valid, parameter type is optional
    listDevs.sort((o1, o2)->o1.getAge()-o2.getAge());

3.2 Sort by name

    //sort by name
    Collections.sort(listDevs, new Comparator() {
        @Override
        public int compare(Developer o1, Developer o2) {
            return o1.getName().compareTo(o2.getName());
        }
    });

    //lambda
    listDevs.sort((Developer o1, Developer o2)->o1.getName().compareTo(o2.getName()));

    //lambda
    listDevs.sort((o1, o2)->o1.getName().compareTo(o2.getName()));

3.3 Sort by salary

    //sort by salary
    Collections.sort(listDevs, new Comparator() {
        @Override
        public int compare(Developer o1, Developer o2) {
            return o1.getSalary().compareTo(o2.getSalary());
        }
    });

    //lambda
    listDevs.sort((Developer o1, Developer o2)->o1.getSalary().compareTo(o2.getSalary()));

    //lambda
    listDevs.sort((o1, o2)->o1.getSalary().compareTo(o2.getSalary()));

3.4 Reversed sorting.

3.4.1 Lambda expression to sort a List using their salary.

    Comparator salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
    listDevs.sort(salaryComparator);

Ausgabe

Developer [name=example, salary=70000, age=33]
Developer [name=alvin, salary=80000, age=20]
Developer [name=jason, salary=100000, age=10]
Developer [name=iris, salary=170000, age=55]

3.4.2 Lambda expression to sort a List using their salary, reversed order.

    Comparator salaryComparator = (o1, o2)->o1.getSalary().compareTo(o2.getSalary());
    listDevs.sort(salaryComparator.reversed());

Ausgabe

Developer [name=iris, salary=170000, age=55]
Developer [name=jason, salary=100000, age=10]
Developer [name=alvin, salary=80000, age=20]
Developer [name=example, salary=70000, age=33]

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Java 8 Lambda: Vergleichsbeispiel