Pattern matching instanceof [closed]

问题

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Closed 5 hours ago.

I came across this amazing topic on https://www.baeldung.com/java-pattern-matching-instanceof. But when I try to run the following code, it throws compile time error:

if(obj instanceof String s) {
    System.out.println(s);
}

I am using Java 14.

回答1:

This is a preview feature in Java 14, see JEP 305 and JEP 375. To enable this, compile your class with:

javac MainClass.java --enable-preview --release 14

And now you can do:

java MainClass --enable-preview

Example of instanceof:

Object o = "Hello World!";
if(o instanceof String s) {
    // no explicit type casting
    s = s.replaceFirst("World", "Java"); // No compile time issues
    System.out.println(s);
}

Another example copied from JEP:

if (obj instanceof String s && s.length() > 5) {.. s.contains(..) ..}

来源：https://stackoverflow.com/questions/61939967/pattern-matching-instanceof