考慮下面寫文件的函數:
這個方法背後的想法是允許用戶傳入InputStream的不同實現方法。所以writeToFile能被
GZIPOuputStream, SnappyOuputStream(快速壓縮)或者只是一個普通的FileInputStream調用。
private static void writeToFile(File file, String value,
Function<OutputStream, OutputStream> writing) throws IOException{
try (PrintWriter pw = new PrintWriter(new BufferedOutputStream
(writing.apply(new FileOutputStream(file))))) {
pw.write(value);
}
這個整潔的方法能被這樣調用
public static void main(String[] args) {
try {
//Write with compression
//DOES NOT COMPILE!!
writeToFile(new File("test"), "Hello World", GZIPOutputStream::new);
//Just use the FileOutputStream
writeToFile(new File("test"), "Hello World", i->i);
}catch(IOException e){
//deal with exception as you choose
}
}
不幸的是正如註釋中所指出的這段代碼編譯不通過。不編譯的原因是因爲GZIPOutputStream在它的構造器裏拋出了一個IOException。如果IOException在lambda之外被拋出,就可以用一個try catch塊來處理。但是lambdas不是這樣工作的。
事實上你這樣處理異常可以讓它正常編譯:
public static void main(String[] args) {
try {
//Write with compression
//COMPILES BUT SO UGLY
writeToFile(new File("test"), "Hello World", i -> {
try {
return new GZIPOutputStream(i);
} catch (IOException e) {
//HOW ARE WE SUPPOSED TO DEAL WITH THIS EXCEPTION??
throw new AssertionError(e);
}
});
//Just use the FileOutputStream
writeToFile(new File("test"), "Hello World", i->i);
}catch(IOException e){
//deal with exception as you choose
}
}
這樣做不僅代碼醜陋,你還留下了一個棘手的問題,就是IOExceptin怎麼處理。在這種情況下我們只要在一個AssertionError內部重新打包。看我上一編文章“欺騙異常”中用正確的方式來處理這種情況。
但是這個問題有一個解決方案。不是用java.util.function.Function 傳一個值和返回一個值,我們能創建一個傳一個值、返回一個值和拋出一個異常的自定義函數。用這種方式的話,
writeToFile的客戶端代碼就非常簡潔乾淨,還能用自然的方式來處理這個異常。此外,lambdas
現在這種使用方式是爲了讓我們的代碼變得更漂亮易懂。
完整代碼清單
package util;
import java.io.*;
import java.util.zip.GZIPOutputStream;
public class LambdaExceptions {
public static void main(String[] args) {
try {
//Write with compression
writeToFile(new File("test"), "Hello World", GZIPOutputStream::new);
//Just use the FileOutputStream
writeToFile(new File("test"), "Hello World", i->i);
}catch(IOException e){
//deal with exception as you choose
}
}
private static void writeToFile(File file, String value,
ThrowingFunction<OutputStream, OutputStream, IOException> writing) throws IOException{
try (PrintWriter pw = new PrintWriter(new BufferedOutputStream
(writing.apply(new FileOutputStream(file))))) {
pw.write(value);
}
}
@FunctionalInterface
public interface ThrowingFunction<I, O, T extends Throwable> {
O apply(I i) throws T;
}
}