我不太确定我是否了解Oracle Java教程中的Lambda表达式教程。令我感到困惑的主要是lambda的Index参数。ds.print(index ->{...}
编译器如何知道什至是什么值索引?索引没有在程序的其他任何地方声明,所以Index参数甚至引用的是什么?编译器如何知道?
有问题的运动:
public class DataStructure {
private final static int SIZE = 15;
private int[] arrayOfInts = new int[SIZE];
public DataStructure() {
for (int i = 0; i < SIZE; i++) {
arrayOfInts[i] = i;
}
}
public int size() {
return SIZE;
}
public int get(int index) {
return arrayOfInts[index];
}
interface DataStructureIterator extends java.util.Iterator<Integer> { }
private class EvenIterator implements DataStructureIterator {
private int nextIndex = 0;
public boolean hasNext() {
return (nextIndex <= SIZE - 1);
}
public Integer next() {
Integer retValue = Integer.valueOf(arrayOfInts[nextIndex]);
nextIndex += 2;
return retValue;
}
}
public DataStructureIterator getEvenIterator() {
return new EvenIterator();
}
public void printEven() {
DataStructureIterator iterator = getEvenIterator();
while (iterator.hasNext()) {
System.out.print(iterator.next() + " ");
}
System.out.println();
}
public void print(DataStructureIterator iterator) {
while (iterator.hasNext()) {
System.out.print(iterator.next() + " ");
}
System.out.println();
}
public void print(java.util.function.Function<Integer, Boolean> function) {
for (int i = 0; i < SIZE; i++) {
if (function.apply(i)) {
System.out.print(arrayOfInts[i] + " ");
}
}
System.out.println();
}
public static Boolean isEvenIndex(Integer index) {
if (index % 2 == 0) return Boolean.TRUE;
return Boolean.FALSE;
}
public static Boolean isOddIndex(Integer index) {
if (index % 2 == 0) return Boolean.FALSE;
return Boolean.TRUE;
}
public static void main(String s[]) {
DataStructure ds = new DataStructure();
System.out.println("printEven()");
ds.printEven();
System.out.println("print(DataStructureIterator) with "
+ "getEvenIterator");
ds.print(ds.getEvenIterator());
System.out.println("print(DataStructureIterator) with "
+ "anonymous class, odd indicies");
ds.print(
new DataStructure.DataStructureIterator() {
private int nextIndex = 1;
public boolean hasNext() {
return (nextIndex <= ds.size() - 1);
}
public Integer next() {
int retValue = ds.get(nextIndex);
nextIndex += 2;
return retValue;
}
}
);
System.out.println("print(Function) with lambda expressions");
ds.print(index -> {
if (index % 2 == 0) return Boolean.TRUE;
return Boolean.FALSE;
});
ds.print(index -> {
if (index % 2 == 0) return Boolean.FALSE;
return Boolean.TRUE;
});
System.out.println("print(Function) with method references");
ds.print(DataStructure::isEvenIndex);
ds.print(DataStructure::isOddIndex);
}
}
该ds.print
方法采用type参数Function<Integer,Boolean>
。所以这:
ds.print(index -> {
if (index % 2 == 0) return Boolean.TRUE;
return Boolean.FALSE;
});
使用匿名类而不是lambda等效于此语法:
ds.print(new Function<Integer,Boolean>() {
@Override
public Boolean apply(Integer index) {
if (index % 2 == 0) return Boolean.TRUE;
return Boolean.FALSE;
}
});
(Function
该类的功能方法是apply
。)
因此,参数名称index
是任意的。您可以随便叫它。它只是lambda方法的本地名称。它的值由以下调用提供print
:
if (function.apply(i)) ...
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