Overview
Overview
We have learnt already about how to implement Command and Observer patterns using Java 8 Lambda Expression. Now, we would look into Strategy Pattern.
Strategy Pattern
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The Problem Domain
The problem is an excerpt from the book Java 8 Lambdas by Richard Warburton.
We have a requirement of compressing a specific file using different compression methods. We need an efficient way to add extensions to add several compressions strategies so that we need not to violate the Open Closed Principle.
OOP way
Let’s first solve it the usual way.
Strategy
public interface CompressionStrategy {
public OutputStream compress(OutputStream data) throws IOException;
}Concrete Strategy - GZIP Compression
public final class GzipCompressionStrategy implements CompressionStrategy {
@Override
public OutputStream compress(final OutputStream data) throws IOException {
return new GZIPOutputStream(data);
}
}Concrete Strategy - ZIP Compression
public final class ZipCompressionStrategy implements CompressionStrategy {
@Override
public OutputStream compress(final OutputStream data) throws IOException {
return new ZipOutputStream(data);
}
}Client
public final class Compressor {
public static void init(final Path inFile, final File outFile) throws IOException {
final Compressor gzipCompressor = new Compressor(new GzipCompressionStrategy());
gzipCompressor.compress(inFile, outFile);
final Compressor zipCompressor = new Compressor(new ZipCompressionStrategy());
zipCompressor.compress(inFile, outFile);
}
private final CompressionStrategy strategy;
public Compressor(final CompressionStrategy strategy) {
this.strategy = strategy;
}
public void compress(final Path inFile, final File outFile) throws IOException {
try (OutputStream outStream = new FileOutputStream(outFile)) {
Files.copy(inFile, this.strategy.compress(outStream));
}
}
}Functional Way
You can assume that it’s gonna be simpler than the previous design patterns. We just have to write the client class to pass the concrete compression strategies as behaviors to Compressor constructor argument as it accepts the SAM (Single Abstract Method) Interface CompressionStrategy.
Client
public final class Compressor {
public static void init(final Path inFile, final File outFile) throws IOException {
final Compressor gzipCompressor = new Compressor(GZIPOutputStream::new);
gzipCompressor.compress(inFile, outFile);
final Compressor zipCompressor = new Compressor(ZipOutputStream::new);
zipCompressor.compress(inFile, outFile);
final Compressor zipCompressor2 = new Compressor(data -> new ZipOutputStream(data));
zipCompressor2.compress(inFile, outFile);
}
private final CompressionStrategy strategy;
public Compressor(final CompressionStrategy strategy) {
this.strategy = strategy;
}
public void compress(final Path inFile, final File outFile) throws IOException {
try (OutputStream outStream = new FileOutputStream(outFile)) {
Files.copy(inFile, this.strategy.compress(outStream));
}
}
}Conclusion
Using Strategy Pattern, the user can change or alter the algorithms that the client application is using.