JDK21新特性Record Patterns记录模式详解(最新推荐)

// ＜Java 16
if (obj instanceof String) {
    String s = (String)obj;
    ... 使用s ...
}
// ≥Java 16
if (obj instanceof String s) {
    ... 使用s ...
}

class Point {
    private int x;
    private int y;
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
    public int getX() {
        return x;
    }
    public int getY() {
        return y;
    }
}
static void printSum(Object obj) {
    if (obj instanceof Point) {
        Point p = (Point) obj;
        int x = p.getX();
        int y = p.getY();
        System.out.println(x + y);
    }
}

record Point(int x, int y) {}
static void printSum(Object obj) {
    if (obj instanceof Point p) {
        int x = p.x();
        int y = p.y();
        System.out.println(x+y);
    }
}

// Java 21及以后
static void printSum(Object obj) {
    if (obj instanceof Point(int x, int y)) {
        System.out.println(x+y);
    }
}

// Java 16及以后
record Point(int x, int y) {}
enum Color { RED, GREEN, BLUE }
record ColoredPoint(Point p, Color c) {}
record Rectangle(ColoredPoint upperLeft, ColoredPoint lowerRight) {}

// Java 21及以后
static void printUpperLeftColoredPoint(Rectangle r) {
    if (r instanceof Rectangle(ColoredPoint ul, ColoredPoint lr)) {
         System.out.println(ul.c());
    }
}

// Java 21及以后
static void printColorOfUpperLeftPoint(Rectangle r) {
    if (r instanceof Rectangle(ColoredPoint(Point p, Color c),
                               ColoredPoint lr)) {
        System.out.println(c);
    }
}

// Java 16及以后
Rectangle r = new Rectangle(new ColoredPoint(new Point(x1, y1), c1), 
                            new ColoredPoint(new Point(x2, y2), c2));

// Java 21及以后
static void printXCoordOfUpperLeftPointWithPatterns(Rectangle r) {
    if (r instanceof Rectangle(ColoredPoint(Point(var x, var y), var c),
                               var lr)) {
        System.out.println("Upper-left corner: " + x);
    }
}

// Java 21及以后
record Pair(Object x, Object y) {}
Pair p = new Pair(42, 42);
if (p instanceof Pair(String s, String t)) {
    System.out.println(s + ", " + t);
} else {
    System.out.println("Not a pair of strings");
}

Pattern:
  TypePattern
  RecordPattern
TypePattern:
  LocalVariableDeclaration
RecordPattern:
  ReferenceType ( [ PatternList ] )
PatternList: 
  Pattern { , Pattern }

record Point(int i, int j) {}

// Java 21及以后
record MyPair<S,T>(S fst, T snd){};
static void recordInference(MyPair<String, Integer> pair){
    switch (pair) {
        case MyPair(var f, var s) -> 
            ... // 推断的记录模式 MyPair<String,Integer>(var f, var s)
        ...
    }
}

// Java 21及以后
record Box<T>(T t) {}
static void test1(Box<Box<String>> bbs) {
    if (bbs instanceof Box<Box<String>>(Box(var s))) {
        System.out.println("String " + s);
    }
}

// Java 21及以后
static void test2(Box<Box<String>> bbs) {
    if (bbs instanceof Box(Box(var s))) {
        System.out.println("String " + s);
    }
}

class A {}
class B extends A {}
sealed interface I permits C, D {}
final class C implements I {}
final class D implements I {}
record Pair<T>(T x, T y) {}
Pair<A> p1;
Pair<I> p2;

// Java 21及以后
switch (p1) {                 // 错误！
    case Pair<A>(A a, B b) -> ...
    case Pair<A>(B b, A a) -> ...
}

// Java 21及以后
switch (p2) {
    case Pair<I>(I i, C c) -> ...
    case Pair<I>(I i, D d) -> ...
}
switch (p2) {
    case Pair<I>(C c, I i) -> ...
    case Pair<I>(D d, C c) -> ...
    case Pair<I>(D d1, D d2) -> ...
}

// Java 21及以后
switch (p2) {                        // 错误！
    case Pair<I>(C fst, D snd) -> ...
    case Pair<I>(D fst, C snd) -> ...
    case Pair<I>(I fst, C snd) -> ...
}