Lambda简化代码

Optional对空值的处理

比如要取user中的一个多层次数据

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public String getCity(User user)  throws Exception{
    if(user!=null){
        if(user.getAddress()!=null){
            Address address = user.getAddress();
            if(address.getCity()!=null){
                return address.getCity();
            }
        }
    }
    throw new Excpetion("取值错误"); 
}

// 可用lambda替换为
public String getCity(User user) throws Exception{
    return Optional.ofNullable(user)
                   .map(u-> u.getAddress())
                   .map(a->a.getCity())
                   .orElseThrow(()->new Exception("取值错误"));
}

Optional对空值的判断处理

判断某个对象是否相等,如果没有则创建新的对象

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public User getUser(User user, String username) throws Exception{
    if(user!=null){
        String name = user.getName();
        if(username.equals(name)){
            return user;
        }
    }else{
        user = new User();
        user.setName(username);
        return user;
    }
}

// 可用lambda替换为
public User getUser(User user, String username) {
    return Optional.ofNullable(user)
                   .filter(u->username.equals(u.getName()))
                   .orElseGet(()-> {
                        User user1 = new User();
                        user1.setName(username);
                        return user1;
                   });
}

用lambda表达式实现Runnable

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// (params) -> expression
// (params) -> statement
// (params) -> { statements } 
// () -> System.out.println("Hello Lambda Expressions");
// (int even, int odd) -> even + odd 

new Thread(new Runnable() {
    @Override
    public void run() {
    System.out.println("Before Java8, too much code for too little to do");
    }
}).start();

// 可用lambda替换为
new Thread( () -> System.out.println("In Java8, Lambda expression rocks !!") ).start();

使用Java 8 lambda表达式进行事件处理

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JButton show =  new JButton("Show");
show.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
    System.out.println("Event handling without lambda expression is boring");
    }
}); 

// 可用lambda替换为
show.addActionListener((e) -> {
    System.out.println("Light, Camera, Action !! Lambda expressions Rocks");
});

使用lambda表达式对列表进行迭代

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List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
for (String feature : features) {
    System.out.println(feature);
} 

// 可用lambda替换为
List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
features.forEach(n -> System.out.println(n));

// 也可以替换为
features.forEach(System.out::println);