策略模式
策略模式是一种常见的在运行时改变代码行为的设计模式。
参见以下例子：
// 策略类接口
package com.chapter.eight.strategy;
import java.io.IOException;
import java.io.OutputStream;
public interface CompressionStrategy {
OutputStream compress(OutputStream data) throws IOException;
}
// 实体策略类1
package com.chapter.eight.strategy;
import java.io.IOException;
import java.io.OutputStream;
import java.util.zip.GZIPOutputStream;
public class GZipCompressionStrategy implements CompressionStrategy {
@Override
public OutputStream compress(OutputStream data) throws IOException {
return new GZIPOutputStream(data);
}
}
// 实体策略类2
package com.chapter.eight.strategy;
import java.io.OutputStream;
import java.util.zip.ZipOutputStream;
public class ZipCompressionStrategy implements CompressionStrategy {
@Override
public OutputStream compress(OutputStream data) {
return new ZipOutputStream(data);
}
}
// 策略使用者
package com.chapter.eight.strategy;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.file.Files;
import java.nio.file.Path;
public class Compressor {
private final CompressionStrategy strategy;
public Compressor(CompressionStrategy strategy) {
this.strategy = strategy;
}
public void compress(Path inFile, File outFile) throws IOException {
try (OutputStream outStream = new FileOutputStream(outFile)) {
Files.copy(inFile, strategy.compress(outStream));
}
}
}
// 入口
package com.chapter.eight.strategy;
public class CompressDemo {
public static void main(String[] args) {
Compressor gzipCompressor = new Compressor(new GzipCompressionStrategy());
Compressor zipCompressor = new Compressor(new ZipCompressionStrategy());
}
}
在上述例子中，GZipCompressionStrategy和ZipCompressionStrategy两个策略实体类都实现了CompressionStrategy接口，后者是一个标准的函数接口，因此一定可以替换为lambda表达式。该函数接口输入一个OutputStream对象，输出一个OutputStream对象，符合GZipOutputStream和ZipOutputStream的构造方法的签名，因此可以用其构造方法代替。
// GZIPOutputStream的构造方法
public GZIPOutputStream(OutputStream out) throws IOException {
this(out, 512, false);
}
// 入口
package com.chapter.eight.strategy;
public class CompressDemo {
public static void main(String[] args) {
Compressor gzipCompressor = new Compressor(GZipOutputStream::new);
Compressor zipCompressor = new Compressor(ZipOutputStream::new));
}
}
可以看到lambda表达式和方法引用可以有效替换策略模式中的具体策略类，给策略的使用者传入不同的行为。
