Java8 方法引用的使用

####一、方法引用分类

11.构造函数
22.静态方法
33.实例方法（类型）
44.实例方法（对象）

####二、方法引用-构造函数 ClassName::new，示例：

1/**
2 * @author Kevin
3 * @date 2017-01-24
4 */
5public class ConstructorTest {
6
7    public static void main(String[] args) {
8        Function<String, String> function = String::new;
9        System.out.println(function.apply("SpaceX"));
10    }
11
12}

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

1/**
2 * @author Kevin
3 * @date 2017-01-24
4 */
5public class StaticMethodTest {
6
7    public static void main(String[] args) {
8        Function<Integer, String> function = String::valueOf;
9        System.out.println(function.apply(1001));
10    }
11
12}

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

1/**
2 * @author Kevin
3 * @date 2017-01-24
4 */
5public class ClassNameInstanceMethodTest {
6
7    public static void main(String[] args) {
8        Function<String, Integer> function = String::length;
9        System.out.println(function.apply("Google"));
10    }
11
12}

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

1/**
2 * 公司
3 *
4 * @author Kevin
5 * @date 2017-01-24
6 */
7public class Company {
8
9    /**
10     * 编码
11     */
12    private String code;
13
14    /**
15     * 公司名称
16     */
17    private String name;
18
19    public Company() {
20
21    }
22
23    public Company(String name, String code) {
24        this.name = name;
25        this.code = code;
26    }
27
28    public static int compareByCode(Company a, Company b) {
29        return a.getCode().compareTo(b.getCode());
30    }
31
32    @Override
33    public String toString() {
34        return "Company{" +
35                "code='" + code + '\'' +
36                ", name='" + name + '\'' +
37                '}';
38    }
39
40    /**
41     * 生产
42     *
43     * @author Kevin
44     */
45    String produce(String productName) {
46        return getName() + " produces " + productName;
47    }
48
49    public String getCode() {
50        return code;
51    }
52
53    public void setCode(String code) {
54        this.code = code;
55    }
56
57    public String getName() {
58        return name;
59    }
60
61    public void setName(String name) {
62        this.name = name;
63    }
64
65}
66
67/**
68 * @author Kevin
69 * @date 2017-01-24
70 */
71public class ObjectInstanceMethodTest {
72
73    public static void main(String[] args) {
74        Company company = new Company();
75        company.setName("Apple");
76        Function<String, String> converter = company::produce;
77        System.out.println(converter.apply("iPhone 7"));
78    }
79
80}

五、方法引用的综合使用

1/**
2 * Java8 方法引用
3 *
4 * @author Kevin
5 * @date 2017-01-24
6 */
7public class MethodReferencesTest {
8
9    public static void main(String[] args) {
10        List<Company> companyList = new ArrayList<>();
11        companyList.add(new Company("Google", "1002"));
12        companyList.add(new Company("SpaceX", "1001"));
13        companyList.add(new Company("Apple", "1003"));
14
15        System.out.println("------companyList------");
16        companyList.forEach(System.out::println);
17
18        Company[] listAsArray = companyList.toArray(new Company[companyList.size()]);
19
20        // 不使用方法引用
21        Arrays.sort(listAsArray, new Comparator<Company>() {
22
23            @Override
24            public int compare(Company a, Company b) {
25                return a.getCode().compareTo(b.getCode());
26            }
27
28        });
29
30        // 使用lambada表达式
31        Arrays.sort(listAsArray,
32                (Company a, Company b) -> {
33                    return a.getCode().compareTo(b.getCode());
34                }
35        );
36
37        // 方法引用(静态方法)
38        Arrays.sort(listAsArray, Company::compareByCode);
39        System.out.println("------listAsArray------");
40        for (int i = 0; i < listAsArray.length; i++) {
41            System.out.println(listAsArray[i]);
42        }
43
44        class ComparisonProvider {
45            public int compareByName(Company a,
46                                     Company b) {
47                return a.getName().compareTo(b.getName());
48            }
49        }
50        ComparisonProvider comparisonProvider = new ComparisonProvider();
51        // 方法引用(实例方法)
52        Arrays.sort(listAsArray, comparisonProvider::compareByName);
53
54        // 使用lambada表达式构造函数
55        Set<Company> companySetLambda =
56                transferElements(companyList, () -> {
57                    return new HashSet<>();
58                });
59        System.out.println("------companySetLambda------");
60        companySetLambda.forEach(System.out::println);
61
62        // 使用new运算符构造函数
63        Set<Company> companySet = transferElements(
64                companyList, HashSet::new);
65        System.out.println("------companySet------");
66        companySet.forEach(System.out::println);
67
68    }
69
70    /**
71     * 将集合中的元素复制到另一个集合
72     *
73     * @author Kevin
74     */
75    public static <T, SOURCE extends Collection<T>, DEST extends Collection<T>>
76    DEST transferElements(
77            SOURCE sourceCollection,
78            Supplier<DEST> collectionFactory) {
79
80        DEST result = collectionFactory.get();
81        for (T t : sourceCollection) {
82            result.add(t);
83        }
84        return result;
85    }
86
87}
88
89/**
90 * 公司
91 *
92 * @author Kevin
93 * @date 2017-01-24
94 */
95public class Company {
96
97    /**
98     * 编码
99     */
100    private String code;
101
102    /**
103     * 公司名称
104     */
105    private String name;
106
107    public Company() {
108
109    }
110
111    public Company(String name, String code) {
112        this.name = name;
113        this.code = code;
114    }
115
116    public static int compareByCode(Company a, Company b) {
117        return a.getCode().compareTo(b.getCode());
118    }
119
120    @Override
121    public String toString() {
122        return "Company{" +
123                "code='" + code + '\'' +
124                ", name='" + name + '\'' +
125                '}';
126    }
127
128    /**
129     * 生产
130     *
131     * @author Kevin
132     */
133    String produce(String productName) {
134        return getName() + " produces " + productName;
135    }
136
137    public String getCode() {
138        return code;
139    }
140
141    public void setCode(String code) {
142        this.code = code;
143    }
144
145    public String getName() {
146        return name;
147    }
148
149    public void setName(String name) {
150        this.name = name;
151    }
152
153}

六、参考文章

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