java 8开始支持lambda，为了方便 函数式编程 ， jdk 在java.util.function包提供了大量可以使用的函数式接口，其中Supplier就是一个常用接口。

一、Supplier

从名字来它是一个数据提供者，只有一个没有入参的get的抽象方法，返回一个T类型的值。

下面是这个接口的源代码：

 package java.util.function;

/**
 * Represents a supplier of results.
 *
 * <p>There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #get()}.
 *
 * @param <T> the type of results supplied by this supplier
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Supplier<T> {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}  

下面举例说明：

例子1：通过lambda来定义一个Supplier

     private static void testBaseFunction() {
        Random random = new Random();
        Supplier< Long > randomSupplier = ()->{
            return random.nextLong();
        };
        System.out.println("random  long :" + randomSupplier.get());
    }  

输出：

 random long:-2470706042123132983  

例子2：通过方法引用来定义一个Supplier

 private static void testMethodReference() {
        // Person必须包含无参构造函数，否则此处会报错 'Cannot resolve constructor 'Person'
        Supplier<Person> personSupplier = Person::new;
        Person person = personSupplier.get();
        // 通过Person的 默认构造函数 来创建Person实例
        person.playWith("Jack Q");
  }

  public static class Person{
        public void playWith(String name){
            System.out.println("I like to play with " + name);
        }
  }  

输出：

 I like to play with  Jack Q  

例子3： forEach

    ivate static void testForEach() {
        List<String> list = Arrays.asList("A","N","C");
        list.forEach(System.out::println);
        list.stream().forEach(System.out::println);
   }  

二、支持返回基础数据类型的Supplier

因为java的泛型是不支持 boolean , int, long, double 等基础数据类型的，所以在jdk的java.util.function包下也提供了4个支持返回基础数据类型的Supplier函数式接口。

1. BooleanSupplier

 package java.util.function;


/**
 * Represents a supplier of {@code boolean}-valued results.  This is the
 * {@code boolean}-producing primitive specialization of {@link Supplier}.
 *
 * <p>There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #getAsBoolean()}.
 *
 * @see Supplier
 * @since 1.8
 */
@FunctionalInterface
public interface BooleanSupplier {

    /**
     * Gets a result.
     *
     * @return a result
     */
    boolean getAsBoolean();
}  

2. DoubleSupplier

 package java.util.function;

/**
 * Represents a supplier of {@code double}-valued results.  This is the
 * {@code double}-producing primitive specialization of {@link Supplier}.
 *
 * <p>There is no requirement that a distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #getAsDouble()}.
 *
 * @see Supplier
 * @since 1.8
 */
@FunctionalInterface
public interface DoubleSupplier {

    /**
     * Gets a result.
     *
     * @return a result
     */
    double getAsDouble();
}  

3. LongSupplier

 package java.util.function;

/**
 * Represents a supplier of {@code long}-valued results.  This is the
 * {@code long}-producing primitive specialization of {@link Supplier}.
 *
 * <p>There is no requirement that a distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #getAsLong()}.
 *
 * @see Supplier
 * @since 1.8
 */
@FunctionalInterface
public interface LongSupplier {

    /**
     * Gets a result.
     *
     * @return a result
     */
    long getAsLong();
}  

4. IntSupplier

 package java.util.function;

/**
 * Represents a supplier of {@code int}-valued results.  This is the
 * {@code int}-producing primitive specialization of {@link Supplier}.
 *
 * <p>There is no requirement that a distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #getAsInt()}.
 *
 * @see Supplier
 * @since 1.8
 */
@FunctionalInterface
public interface IntSupplier {

    /**
     * Gets a result.
     *
     * @return a result
     */
    int getAsInt();
}