// 1. 基礎用法
Function parser = Integer::parseInt;
Integer result = parser.apply("123"); // 123
// 2. 多參數靜態方法
BinaryOperator power = Math::pow;
Double result = power.apply(2.0, 3.0); // 8.0
// 3. 集合工具類方法引用 (需匹配函數式接口)
List numbers = Arrays.asList(1, 10, 3, 5);
Function, Integer> maxFn = Collections::max;
Integer max = maxFn.apply(numbers); // 10
// 等價 Lambda
Function, Integer> maxFnLambda = list -> Collections.max(list);
實戰案例：字符串反轉
@FunctionalInterface
interface StringFunc {
String func(String n);
}
class MyStringOps {
// 靜態方法
public static String strReverse(String str) {
StringBuilder sb = new StringBuilder(str);
return sb.reverse().toString();
}
}
class MethodRefDemo {
public static String stringOp(StringFunc sf, String s) {
return sf.func(s);
}
public static void main(String[] args) {
String inStr = "lambda add power to Java";
// MyStringOps::strReverse 實現了 StringFunc.func()
String outStr = stringOp(MyStringOps::strReverse, inStr);
System.out.println("Original: " + inStr);
System.out.println("Reversed: " + outStr);
}
}
// 輸出: Original: lambda add power to Java
// Reversed: avaJ ot rewop dda adbmal
實戰案例：自定義比較器
class MyClass {
private int val;
MyClass(int v) { val = v; }
public int getValue() { return val; }
}
class UseMethodRef {
// 靜態比較方法
public static int compareMC(MyClass a, MyClass b) {
return a.getValue() - b.getValue();
}
public static void main(String[] args) {
List list = Arrays.asList(
new MyClass(1), new MyClass(4), new MyClass(2),
new MyClass(9), new MyClass(3), new MyClass(7)
);
// Collections.max 需要 Comparator
// UseMethodRef::compareMC 自動適配為 Comparator
MyClass max = Collections.max(list, UseMethodRef::compareMC);
System.out.println("Max value: " + max.getValue()); // 9
}
}
2. 實例方法引用 (三種子類型)
a) 特定對象的實例方法引用
語法：instanceReference::methodName
class MyStringOps {
// 實例方法
public String strReverse(String str) {
return new StringBuilder(str).reverse().toString();
}
}
public class Demo {
public static void main(String[] args) {
MyStringOps ops = new MyStringOps(); // 具體實例
StringFunc sf = ops::strReverse; // 引用該實例的方法
System.out.println(sf.func("Hello")); // olleH
}
}
b) 任意對象的實例方法引用 (類型上的實例方法)
語法：ClassName::instanceMethodName
關鍵規則：函數式接口第一個參數成為方法調用者 (this)，後續參數映射為方法參數。
@FunctionalInterface
interface MyFunc {
boolean func(T v1, T v2);
}
class HighTemp {
private int hTemp;
HighTemp(int ht) { hTemp = ht; }
// 實例方法：比較溫度
public boolean sameTemp(HighTemp ht2) { return hTemp == ht2.hTemp; }
public boolean lessThanTemp(HighTemp ht2) { return hTemp < ht2.hTemp; }
}
public class InstanceMethDemo {
// 計數器：統計滿足條件的元素個數
public static int counter(T[] vals, MyFunc f, T v) {
int count = 0;
for (T val : vals) if (f.func(val, v)) count++;
return count;
}
public static void main(String[] args) {
HighTemp[] week = {
new HighTemp(89), new HighTemp(82), new HighTemp(90),
new HighTemp(89), new HighTemp(89), new HighTemp(91),
new HighTemp(84), new HighTemp(83)
};
// HighTemp::sameTemp → (v1, v2) -> v1.sameTemp(v2)
int count = counter(week, HighTemp::sameTemp, new HighTemp(89));
System.out.println(count + " days had high of 89"); // 3
count = counter(week, HighTemp::lessThanTemp, new HighTemp(89));
System.out.println(count + " days less than 89"); // 3
}
}
