java8中的方法引用與構造器引用

java8中的方法引用與構造器引用

方法引用：若Lambda體中的內容有方法已經實現了，我們可以使用“方法引用”

主要的三種語法格式：

1. 對象::實例名
2. 類::靜態方法名
3. 類::實例方法名

注意：

1. Lmabda體中調用方法的參數列表與返回值類型要與函數式接口中抽象方法的函數列表和返回值類型保持一致

2. 若Lambda參數列表中的第一參數是 實例方法的調用者，而第二個參數是 實例方法的參數時，可以使用ClassName::method

   public class Employee {
   
   private String name;
   private int age;
   private double salary;
   
   public Employee() {
       super();
   }
   
   public  Employee(int age){
       this.age = age;
   }
   
   public Employee(String name, int age, double salary) {
       super();
       this.name = name;
       this.age = age;
       this.salary = salary;
   }
   
   public String getName() {
       return name;
   }
   
   public void setName(String name) {
       this.name = name;
   }
   
   public int getAge() {
       return age;
   }
   
   public void setAge(int age) {
       this.age = age;
   }
   
   public double getSalary() {
       return salary;
   }
   
   public void setSalary(double salary) {
       this.salary = salary;
   }
   
   @Override
   public String toString() {
       return "Employee{" +
               "name='" + name + '\'' +
               ", age=" + age +
               ", salary=" + salary +
               '}';
   }
   }

   //對象::實例方法名
   @Test
   public void test1(){
       PrintStream ps1 = System.out;
       Consumer<String> con = (x) -> ps1.println(x);
   
       PrintStream ps = System.out;
       Consumer<String> con1 = ps::println;
   
       Consumer<String> con2 = System.out::println;
       con2.accept("abcdef");
   }
   
   @Test
   public void test2(){
       Employee employee = new Employee();
       Supplier<String> sup = () -> employee.getName();
       String str = sup.get();
       System.out.println(str);
   
       Supplier<String> sup2 = employee::getName;
       String str2 = sup2.get();
       System.out.println(str2);
   
   }

   //類：靜態方法名
   @Test
   public void test3(){
       Comparator<Integer> com = (x, y) -> Integer.compare(x, y);
   
       Comparator<Integer> com1 = Integer::compare;
   }

   構造器引用

   格式：ClassName::new
   注意：需要調用的構造器的參數列表要與函數式接口中抽象方法的參數列表保持一致

   public class Employee {
   
   private String name;
   private int age;
   private double salary;
   
   public Employee() {
       super();
   }
   
   public  Employee(int age){
       this.age = age;
   }
   
   public Employee(String name, int age, double salary) {
       super();
       this.name = name;
       this.age = age;
       this.salary = salary;
   }
   
   public String getName() {
       return name;
   }
   
   public void setName(String name) {
       this.name = name;
   }
   
   public int getAge() {
       return age;
   }
   
   public void setAge(int age) {
       this.age = age;
   }
   
   public double getSalary() {
       return salary;
   }
   
   public void setSalary(double salary) {
       this.salary = salary;
   }
   
   @Override
   public String toString() {
       return "Employee{" +
               "name='" + name + '\'' +
               ", age=" + age +
               ", salary=" + salary +
               '}';
   }
   }

   //構造器引用
   @Test
   public void test5(){
       Supplier<Employee> sup = () -> new Employee();
   
       //構造器引用方式
       Supplier<Employee> sup2 = Employee::new;
       Employee employee = sup2.get();
       System.out.println(employee);
   }
   
   @Test
   public void test6(){
       Function<Integer, Employee> fun = (x) -> new Employee(x);
   
       Function<Integer, Employee> fun2 = Employee::new;
       Employee emp = fun2.apply(101);
       System.out.println(emp);
   
   }

   數組引用

   格式：Type[ ]::new;

   //數組引用
   @Test
   public void test7(){
       Function<Integer, String[]> fun = (x) -> new String[x];
       String[] strs = fun.apply(10);
       System.out.println(strs.length);
   
       Function<Integer, String[]> fun2 = String[]::new;
       String[] strs2 = fun2.apply(20);
       System.out.println(strs2.length);
   }