Java8 方法引用的使用

####一、方法引用分类

1.构造函数
2.静态方法
3.实例方法（类型）
4.实例方法（对象）

####二、方法引用-构造函数 ClassName::new，示例：

/**
 * @author Kevin
 * @date 2017-01-24
 */
public class ConstructorTest {

    public static void main(String[] args) {
        Function<String, String> function = String::new;
        System.out.println(function.apply("SpaceX"));
    }

}

####三、方法引用-静态方法 ClassName::staticMethodName，示例：

/**
 * @author Kevin
 * @date 2017-01-24
 */
public class StaticMethodTest {

    public static void main(String[] args) {
        Function<Integer, String> function = String::valueOf;
        System.out.println(function.apply(1001));
    }

}

####三、方法引用-实例方法（类型） ClassName::instanceMethodName，示例：

/**
 * @author Kevin
 * @date 2017-01-24
 */
public class ClassNameInstanceMethodTest {

    public static void main(String[] args) {
        Function<String, Integer> function = String::length;
        System.out.println(function.apply("Google"));
    }

}

####四、方法引用-实例方法（对象） object::instanceMethodName，示例：

/**
 * 公司
 *
 * @author Kevin
 * @date 2017-01-24
 */
public class Company {

    /**
     * 编码
     */
    private String code;

    /**
     * 公司名称
     */
    private String name;

    public Company() {

    }

    public Company(String name, String code) {
        this.name = name;
        this.code = code;
    }

    public static int compareByCode(Company a, Company b) {
        return a.getCode().compareTo(b.getCode());
    }

    @Override
    public String toString() {
        return "Company{" +
                "code='" + code + '\'' +
                ", name='" + name + '\'' +
                '}';
    }

    /**
     * 生产
     *
     * @author Kevin
     */
    String produce(String productName) {
        return getName() + " produces " + productName;
    }

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

/**
 * @author Kevin
 * @date 2017-01-24
 */
public class ObjectInstanceMethodTest {

    public static void main(String[] args) {
        Company company = new Company();
        company.setName("Apple");
        Function<String, String> converter = company::produce;
        System.out.println(converter.apply("iPhone 7"));
    }

}

五、方法引用的综合使用

/**
 * Java8 方法引用
 *
 * @author Kevin
 * @date 2017-01-24
 */
public class MethodReferencesTest {

    public static void main(String[] args) {
        List<Company> companyList = new ArrayList<>();
        companyList.add(new Company("Google", "1002"));
        companyList.add(new Company("SpaceX", "1001"));
        companyList.add(new Company("Apple", "1003"));

        System.out.println("------companyList------");
        companyList.forEach(System.out::println);

        Company[] listAsArray = companyList.toArray(new Company[companyList.size()]);

        // 不使用方法引用
        Arrays.sort(listAsArray, new Comparator<Company>() {

            @Override
            public int compare(Company a, Company b) {
                return a.getCode().compareTo(b.getCode());
            }

        });

        // 使用lambada表达式
        Arrays.sort(listAsArray,
                (Company a, Company b) -> {
                    return a.getCode().compareTo(b.getCode());
                }
        );

        // 方法引用(静态方法)
        Arrays.sort(listAsArray, Company::compareByCode);
        System.out.println("------listAsArray------");
        for (int i = 0; i < listAsArray.length; i++) {
            System.out.println(listAsArray[i]);
        }

        class ComparisonProvider {
            public int compareByName(Company a,
                                     Company b) {
                return a.getName().compareTo(b.getName());
            }
        }
        ComparisonProvider comparisonProvider = new ComparisonProvider();
        // 方法引用(实例方法)
        Arrays.sort(listAsArray, comparisonProvider::compareByName);

        // 使用lambada表达式构造函数
        Set<Company> companySetLambda =
                transferElements(companyList, () -> {
                    return new HashSet<>();
                });
        System.out.println("------companySetLambda------");
        companySetLambda.forEach(System.out::println);

        // 使用new运算符构造函数
        Set<Company> companySet = transferElements(
                companyList, HashSet::new);
        System.out.println("------companySet------");
        companySet.forEach(System.out::println);

    }

    /**
     * 将集合中的元素复制到另一个集合
     *
     * @author Kevin
     */
    public static <T, SOURCE extends Collection<T>, DEST extends Collection<T>>
    DEST transferElements(
            SOURCE sourceCollection,
            Supplier<DEST> collectionFactory) {

        DEST result = collectionFactory.get();
        for (T t : sourceCollection) {
            result.add(t);
        }
        return result;
    }

}

/**
 * 公司
 *
 * @author Kevin
 * @date 2017-01-24
 */
public class Company {

    /**
     * 编码
     */
    private String code;

    /**
     * 公司名称
     */
    private String name;

    public Company() {

    }

    public Company(String name, String code) {
        this.name = name;
        this.code = code;
    }

    public static int compareByCode(Company a, Company b) {
        return a.getCode().compareTo(b.getCode());
    }

    @Override
    public String toString() {
        return "Company{" +
                "code='" + code + '\'' +
                ", name='" + name + '\'' +
                '}';
    }

    /**
     * 生产
     *
     * @author Kevin
     */
    String produce(String productName) {
        return getName() + " produces " + productName;
    }

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

六、参考文章

1.Method References (http://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html)