Java 8 奇淫异巧

字符串

连接字符串

String.join(":", "foobar", "foo", "bar");
// => foobar:foo:bar

使用正则表达式过滤

Pattern pattern = Pattern.compile(".*@gmail\\.com");
Stream.of("bob@gmail.com", "alice@hotmail.com")
    .filter(pattern.asPredicate())
    .count();
// =>

数字

Java的Integer是带符号的。所以范围-2^31 ~ 2^31-1
Java8之前超过这个范围会抛出异常

try {
    Integer.parseInt(string, 10);
}
catch (NumberFormatException e) {
    System.err.println("could not parse signed int of " + maxUnsignedInt);
}

Java 8 Integer 可以保存无符号的整型，但是必须通过toUnsignedString才能看到正确的指。

long maxUnsignedInt = (1l << 32) - 1;//4294967295
String string = String.valueOf(maxUnsignedInt);
int unsignedInt = Integer.parseUnsignedInt(string, 10);//-1
String string2 = Integer.toUnsignedString(unsignedInt, 10);//4294967295

数学运算

Java 8增加了基本运算的Exact()方法。
当运算溢出时，抛出异常。

普通加法

System.out.println(Integer.MAX_VALUE);      // 2147483647
System.out.println(Integer.MAX_VALUE + 1);  // -2147483648 溢出

Java 8

try {
    Math.addExact(Integer.MAX_VALUE, 1);
}
catch (ArithmeticException e) {
    System.err.println(e.getMessage());
    // => integer overflow
}

文件

Files.list()

try (Stream<Path> stream = Files.list(Paths.get(""))) {
    String joined = stream
        .map(String::valueOf)
        .filter(path -> !path.startsWith("."))
        .sorted()
        .collect(Collectors.joining("; "));
    System.out.println("List: " + joined);
}

Files.find()

Path start = Paths.get("");
int maxDepth = 5;
try (Stream<Path> stream = Files.find(start, maxDepth, (path, attr) ->
        String.valueOf(path).endsWith(".js"))) {
    String joined = stream
        .sorted()
        .map(String::valueOf)
        .collect(Collectors.joining("; "));
    System.out.println("Found: " + joined);
}

Files.walk()

Path start = Paths.get("");
int maxDepth = 5;
try (Stream<Path> stream = Files.walk(start, maxDepth)) {
    String joined = stream
        .map(String::valueOf)
        .filter(path -> path.endsWith(".js"))
        .sorted()
        .collect(Collectors.joining("; "));
    System.out.println("walk(): " + joined);
}

Read and write files
读整个文件，效率低

List<String> lines = Files.readAllLines(Paths.get("res/nashorn1.js"));
lines.add("print('foobar');");
Files.write(Paths.get("res/nashorn1-modified.js"), lines);

一行一行读

try (Stream<String> stream = Files.lines(Paths.get("res/nashorn1.js"))) {
    stream
        .filter(line -> line.contains("print"))
        .map(String::trim)
        .forEach(System.out::println);
}

使用缓冲

Path path = Paths.get("res/nashorn1.js");
try (BufferedReader reader = Files.newBufferedReader(path)) {
    System.out.println(reader.readLine());
}

Path path = Paths.get("res/output.js");
try (BufferedWriter writer = Files.newBufferedWriter(path)) {
    writer.write("print('Hello World');");
}

或者

Path path = Paths.get("res/nashorn1.js");
try (BufferedReader reader = Files.newBufferedReader(path)) {
    long countPrints = reader
        .lines()
        .filter(line -> line.contains("print"))
        .count();
    System.out.println(countPrints);
}