[Java 8]Lambda學習筆記

在學習Lambda的時候, 我參考的文章是:https://www.cnblogs.com/coprince/p/8692972.html

其中有一個示例是

public static void main(String[] args){
        List<String> languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");

        System.out.println("Languages which starts with J :");
        filter(languages, (str)->str.startsWith("J"));

        System.out.println("Languages which ends with a ");
        filter(languages, (str)->str.endsWith("a"));

        System.out.println("Print all languages :");
        filter(languages, (str)->true);

        System.out.println("Print no language : ");
        filter(languages, (str)->false);

        System.out.println("Print language whose length greater than 4:");
        filter(languages, (str)->str.length() > 4);
    }

    public static void filter(List names, Predicate condition) {
        names.stream().filter((name) -> (condition.test(name))).forEach((name) -> {
        System.out.println(name + " ");
        });
    }

 

但是上面代碼在IDEA中報錯. 提示startsWith, endsWith, length等函數找不到.

然後我在main函數裏通過強制類型轉換, 使得程序可以運行.

代碼如下.

public static void main(String[] args){
        List<String> languages = Arrays.asList("Java", "Scala", "C++", "Haskell", "Lisp");

        System.out.println("Languages which starts with J :");
        filter(languages, str->((String)str).startsWith("J"));

        System.out.println("Languages which ends with a ");
        filter(languages, (str)->((String)str).endsWith("a"));

        System.out.println("Print all languages :");
        filter(languages, (str)->true);

        System.out.println("Print no language : ");
        filter(languages, (str)->false);

        System.out.println("Print language whose length greater than 4:");
        filter(languages, (str)->((String)str).length() > 4);
    }

此時雖然程序可以運行, 但是提示3個告警:

Unchecked call to 'test(T)' as a member of raw type 'java.util.function.Predicate'
Lambda can be replaced with call qualifier more... (Ctrl+F1)

Lambda can be replaced with method reference more... (Ctrl+F1)

進一步修改代碼如下, 

    public static void filter(List<String> names, Predicate condition) {

        names.stream().filter(condition).forEach(System.out::println);
    }

此時告警減少至一個. 進一步修改代碼如下, filter函數告警消失.

public static void filter(List<String> names, Predicate<String> condition) {

    names.stream().filter(condition).forEach(System.out::println);
}

但此時main函數提示String強制類型轉換多餘. 刪除強制類型轉換後, 全部告警消失.

 

總結: 寫代碼時多看函數定義.

比如Steam類的filter函數的本來就傳的Predicate類型, 所以根本不需要重新寫condition的lambda表達式.

再比如, Predicate類本來就是泛型的, 所以應該帶上類型String.

public interface Predicate<T>