java8 新特性 Optional容器類
阿新 • • 發佈:2019-01-26
system ava turn fun 需求 創建 clas nbsp 老師
public class Godness { private String name; public Godness() { } public Godness(String name) { this.name = name; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override publicString toString() { return "Godness [name=" + name + "]"; } }
public class Man { private Godness god; public Man() { } public Man(Godness god) { this.god = god; } public Godness getGod() { return god; } public void setGod(Godness god) {this.god = god; } @Override public String toString() { return "Man [god=" + god + "]"; } }
import java.util.Optional; //註意:Optional 不能被序列化 public class NewMan { private Optional<Godness> godness = Optional.empty(); private Godness god; publicOptional<Godness> getGod(){ return Optional.of(god); } public NewMan() { } public NewMan(Optional<Godness> godness) { this.godness = godness; } public Optional<Godness> getGodness() { return godness; } public void setGodness(Optional<Godness> godness) { this.godness = godness; } @Override public String toString() { return "NewMan [godness=" + godness + "]"; } }
import java.util.Optional; import org.junit.Test; /* * 一、Optional 容器類:用於盡量避免空指針異常 * Optional.of(T t) : 創建一個 Optional 實例 * Optional.empty() : 創建一個空的 Optional 實例 * Optional.ofNullable(T t):若 t 不為 null,創建 Optional 實例,否則創建空實例 * isPresent() : 判斷是否包含值 * orElse(T t) : 如果調用對象包含值,返回該值,否則返回t * orElseGet(Supplier s) :如果調用對象包含值,返回該值,否則返回 s 獲取的值 * map(Function f): 如果有值對其處理,並返回處理後的Optional,否則返回 Optional.empty() * flatMap(Function mapper):與 map 類似,要求返回值必須是Optional */ public class TestOptional { @Test public void test4(){ Optional<Employee> op = Optional.of(new Employee(101, "張三", 18, 9999.99)); Optional<String> op2 = op.map(Employee::getName); System.out.println(op2.get()); Optional<String> op3 = op.flatMap((e) -> Optional.of(e.getName())); System.out.println(op3.get()); } @Test public void test3(){ Optional<Employee> op = Optional.ofNullable(new Employee()); if(op.isPresent()){ System.out.println(op.get()); } Employee emp = op.orElse(new Employee("張三")); System.out.println(emp); Employee emp2 = op.orElseGet(() -> new Employee()); System.out.println(emp2); } @Test public void test2(){ /*Optional<Employee> op = Optional.ofNullable(null); System.out.println(op.get());*/ // Optional<Employee> op = Optional.empty(); // System.out.println(op.get()); } @Test public void test1(){ Optional<Employee> op = Optional.of(new Employee()); Employee emp = op.get(); System.out.println(emp); } @Test public void test5(){ Man man = new Man(); String name = getGodnessName(man); System.out.println(name); } //需求:獲取一個男人心中女神的名字 public String getGodnessName(Man man){ if(man != null){ Godness g = man.getGod(); if(g != null){ return g.getName(); } } return "蒼老師"; } //運用 Optional 的實體類 @Test public void test6(){ Optional<Godness> godness = Optional.ofNullable(new Godness("林誌玲")); Optional<NewMan> op = Optional.ofNullable(new NewMan(godness)); String name = getGodnessName2(op); System.out.println(name); } public String getGodnessName2(Optional<NewMan> man){ return man.orElse(new NewMan()) .getGodness() .orElse(new Godness("蒼老師")) .getName(); } }
java8 新特性 Optional容器類