Java 12 它不是一个长久支持（LTS）版本。

重点特性列表：

• Switch 表达式（预览特性）
• 文件对比增强

Switch 表达式（预览特性）

在 Java 12 中，对 Switch 表达式的写法进行了改进，虽然是一个语法糖的改进，也让 Switch 的代码编写变得更加优雅,如再Java12之前的switch:

        String day = "march";
        switch (day) {
            case "march":
            case "april":
            case "may":
                System.out.println("春天");
                break;
            case "june":
            case "july":
            case "august":
                System.out.println("夏天");
                break;
            case "september":
            case "october":
            case "november":
                System.out.println("秋天");
                break;
            case "december":
            case "january":
            case "february":
                System.out.println("冬天");
                break;
        }

Java12or14现在可以改为如下写法：

        String day = "march";
        switch (day) {
            case "march", "april", "may" -> System.out.println("春天");
            case "june", "july", "august" -> System.out.println("夏天");
            case "september", "october", "november" -> System.out.println("秋天");
            case "december", "january", "february" -> System.out.println("冬天");
        }

以及在表达式返回值,像下面这样：

        String day = "march";
        String data = switch (day) {
            case "march", "april", "may" -> "春天";
            case "june", "july", "august" -> "夏天";
            case "september", "october", "november" -> "秋天";
            case "december", "january", "february" -> "冬天";
            default -> "dayError";
        };
        System.out.println("当前季节是:" + data);

文件对比增强-Files.mismatch

在 Java 中对于文件的操作已经在 Java 7 中进行了一次增强，这次 Java 12 又带来了文件对比功能。对比两个文件，如果内容一致，会返回 -1 ，如果内容不同，会返回不同的字节开始位置。

        // 创建两个文件
        Path pathA = Files.createFile(Paths.get("glj1.txt"));
        Path pathB = Files.createFile(Paths.get("glj2.txt"));

        // 写入相同内容
        Files.write(pathA,"abc".getBytes(), StandardOpenOption.WRITE);
        Files.write(pathB,"abc".getBytes(), StandardOpenOption.WRITE);
        long mismatch = Files.mismatch(pathA, pathB);
        System.out.println(mismatch);

        // 追加不同内容
        Files.write(pathA,"123".getBytes(), StandardOpenOption.APPEND);
        Files.write(pathB,"321".getBytes(), StandardOpenOption.APPEND);
        mismatch = Files.mismatch(pathA, pathB);
        System.out.println(mismatch);

        // 删除创建的文件
        pathA.toFile().deleteOnExit();
        pathB.toFile().deleteOnExit();