Java 8 Supplier Examples

By mkyong | Updated: May 18, 2020

Viewed: 60,934 (+493 pv/w)

Tags: factory functional interface java 8 supplier

In Java 8, Supplier is a functional interface; it takes no arguments and returns a result.

@FunctionalInterface
public interface Supplier<T> {
    T get();
}

1. Supplier

1.1 This example uses Supplier to return a current date-time.

package com.mkyong;

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.function.Supplier;

public class Java8Supplier1 {

    private static final DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

    public static void main(String[] args) {

        Supplier<LocalDateTime> s = () -> LocalDateTime.now();
        LocalDateTime time = s.get();

        System.out.println(time);

        Supplier<String> s1 = () -> dtf.format(LocalDateTime.now());
        String time2 = s1.get();

        System.out.println(time2);

    }

}

Output

2020-03-02T16:10:49.281223
2020-03-02 16:10:49

2. Returns a Supplier

package com.mkyong;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Supplier;

public class Java8Supplier2<T> {

    public static void main(String[] args) {

        Java8Supplier2<String> obj = new Java8Supplier2();

        List<String> list = obj.supplier().get();

    }

    public Supplier<List<T>> supplier() {

        // lambda
        // return () -> new ArrayList<>();

        // constructor reference
        return ArrayList::new;

    }

}

3. Factory

3.1 A simple factory method to return a Developer object.

package com.mkyong;

import java.math.BigDecimal;
import java.time.LocalDate;
import java.util.function.Supplier;

public class Java8Supplier3 {

    public static void main(String[] args) {

        Developer obj = factory(Developer::new);
        System.out.println(obj);

        Developer obj2 = factory(() -> new Developer("mkyong"));
        System.out.println(obj2);

    }

    public static Developer factory(Supplier<? extends Developer> s) {

        Developer developer = s.get();
        if (developer.getName() == null || "".equals(developer.getName())) {
            developer.setName("default");
        }
        developer.setSalary(BigDecimal.ONE);
        developer.setStart(LocalDate.of(2017, 8, 8));

        return developer;

    }

}

package com.mkyong;

import java.math.BigDecimal;
import java.time.LocalDate;

public class Developer {

    String name;
    BigDecimal salary;
    LocalDate start;

    // for factory(Developer::new);
    public Developer() {
    }

    // for factory(() -> new Developer("mkyong"));
    public Developer(String name) {
        this.name = name;
    }

    // get, set, constructor, toString
    //...

}

Output

Developer{name='default', salary=1, start=2017-08-08}
Developer{name='mkyong', salary=1, start=2017-08-08}

References

• Supplier
• Java 8 – How to format LocalDateTime
• DateTimeFormatter JavaDoc
• Java 8 Tutorials