了解何时以及如何使用Java 8 Lambdas [英] Understanding when and how to use Java 8 Lambdas
问题描述
我花了一些时间来学习Java 8的一些新功能。作为练习,我使用一些Java 8功能接口编写了一个MergeSort。我在下面包含完整的代码(可能存在错误/优化,如果它们与Java 8功能相关,我只对它们感兴趣)。我的问题是,我相信在使用我的功能界面时有机会利用lambda表达式,但它还没有点击我的大脑。感觉就像每次我打电话申请,我应该有一种方法可以使用 - >代替。有人可以给我看灯吗?
I have been spending some time trying to learn some of Java 8's new features. As an exercise, I wrote a MergeSort using some Java 8 Functional Interfaces. I'm including the full code below (bugs/optimizations may exist, I am only interested in them if they relate to Java 8 features). My question is, I believe there are opportunities to utilize lambda expressions when I am using my functional interfaces, but it's just not clicking in my brain yet. It feels like everytime I am calling apply, there should be a way I can use "->" instead. Can someone please show me the light?
public class Merge implements BinaryOperator<int[]>{
@Override
public int[] apply(int[] t, int[] u) {
int[] result = new int[t.length + u.length];
for (int i = 0, j = 0, k = 0; i < result.length; i++){
if( j == t.length){
result[i] = u[k++];
} else if (k == u.length) {
result[i] = t[j++];
} else {
result[i] = t[j] < u [k] ? t[j++] : u[k++];
}
}
return result;
}
}
将MergeSort写成java.util .function.Function
MergeSort written as a java.util.function.Function
public class MergeSort implements Function<int[], int[]>{
Merge merge = new Merge();
@Override
public int[] apply(int[] t) {
if(t.length <= 1){
return t;
}
return merge.apply( apply(Arrays.copyOfRange(t, 0, t.length / 2)),
apply(Arrays.copyOfRange(t, t.length / 2, t.length )));
}
}
主要有一个简单的测试用例
Main with one simple test case
public class MergeSortMain {
public static void main(String[] args) {
int values[] = {3,12,6,7,2,1,23,4,5,7,8,4,2,5,365};
MergeSort mergeSort = new MergeSort();
System.out.println(Arrays.toString(mergeSort.apply(values)));
}
}
产生
[1, 2, 2, 3, 4, 4, 5, 5, 6, 7, 7, 8, 12, 23, 365]
推荐答案
lambda表达式的概念是代替创建一个类实现了一个功能接口,你可以定义该接口类型的lambda表达式。
The idea of lambda expressions is that instead of creating a class that implements a functional interface, you can define a lambda expression of the type of that interface.
例如,你的合并
类实现 BinaryOperator< int []>
并且可以替换为以下lambda表达式:
For example, your Merge
class implements BinaryOperator<int[]>
and can be replaced by the following lambda expression :
BinaryOperator<int[]> merge = (t,u) -> {
int[] result = new int[t.length + u.length];
for (int i = 0, j = 0, k = 0; i < result.length; i++){
if( j == t.length){
result[i] = u[k++];
} else if (k == u.length) {
result[i] = t[j++];
} else {
result[i] = t[j] < u [k] ? t[j++] : u[k++];
}
}
return result;
};
现在我们可以类似地创建一个lambda表达式来替换 MergeSort
class,并且,结合两个lambdas,我们得到:
Now we can similarly create a lambda expression to replace the MergeSort
class, and, combining the two lambdas, we get :
public class MergeSortMain {
public static Function<int[], int[]> mergeSort;
public static void main(String[] args) {
int values[] = {3,12,6,7,2,1,23,4,5,7,8,4,2,5,365};
mergeSort = l -> {
BinaryOperator<int[]> merge = (t,u) -> {
int[] result = new int[t.length + u.length];
for (int i = 0, j = 0, k = 0; i < result.length; i++){
if( j == t.length){
result[i] = u[k++];
} else if (k == u.length) {
result[i] = t[j++];
} else {
result[i] = t[j] < u [k] ? t[j++] : u[k++];
}
}
return result;
};
if(l.length <= 1){
return l;
}
return merge.apply( mergeSort.apply(Arrays.copyOfRange(l, 0, l.length / 2)),
mergeSort.apply(Arrays.copyOfRange(l, l.length / 2, l.length )));
};
System.out.println(Arrays.toString(mergeSort.apply(values)));
}
}
关于此代码的一些要点:
Some points regarding this code :
- 我必须从
t $重命名
mergeSort
lambda的参数c $ c>到l
,因为t
也用于merge
lambda。 - 我必须将
mergeSort
lambda声明为静态成员(在分配其值之前),因为它包含对自身的递归调用。
- I had to rename the parameter of
mergeSort
lambda fromt
tol
, sincet
is also used in themerge
lambda. - I had to declare the
mergeSort
lambda as a static member (prior to assigning its value), since it contains recursive calls to itself.
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