在学习Lambda的时候, 我参考的文章是:https://www.cnblogs.com/coprince/p/8692972.html
其中有一个示例是
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 + " ");
});
}
但是上面代码在IDEA中报错. 提示startsWith, endsWith, length等函数找不到.
然后我在main函数里通过强制类型转换, 使得程序可以运行.
代码如下.
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);
}
此时虽然程序可以运行, 但是提示3个告警:
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)
进一步修改代码如下,
public static void filter(List<String> names, Predicate condition) {
names.stream().filter(condition).forEach(System.out::println);
}
此时告警减少至一个. 进一步修改代码如下, filter函数告警消失.
public static void filter(List<String> names, Predicate<String> condition) { names.stream().filter(condition).forEach(System.out::println); }
但此时main函数提示String强制类型转换多余. 删除强制类型转换后, 全部告警消失.
总结: 写代码时多看函数定义.
比如Steam类的filter函数的本来就传的Predicate类型, 所以根本不需要重新写condition的lambda表达式.
再比如, Predicate类本来就是泛型的, 所以应该带上类型String.
public interface Predicate<T>