checker/tests/interning/StringIntern.java

import java.util.HashMap;
import java.util.Map;
import org.checkerframework.checker.interning.qual.*;

public class StringIntern {

  // It would be very handy (and would eliminate quite a few annotations)
  // if any final variable that is initialized to something interned
  // (essentially, to a literal) were treated as implicitly @Interned.
  final String finalStringInitializedToInterned = "foo"; // implicitly @Interned
  final String finalString2 = new String("foo");
  static final String finalStringStatic1 = "foo"; // implicitly @Interned
  static final String finalStringStatic2 = new String("foo");

  static class HasFields {
    static final String finalStringStatic3 = "foo"; // implicitly @Interned
    static final String finalStringStatic4 = new String("foo");
  }

  static class Foo {
    private static Map<Foo, @Interned Foo> pool = new HashMap<>();

    @SuppressWarnings("interning")
    public @Interned Foo intern() {
      if (!pool.containsKey(this)) {
        pool.put(this, (@Interned Foo) this);
      }
      return pool.get(this);
    }
  }

  // Another example of the "final initialized to interned" rule
  final Foo finalFooInitializedToInterned = new Foo().intern();

  public void test(@Interned String arg) {
    String notInternedStr = new String("foo");
    @Interned String internedStr = notInternedStr.intern();
    internedStr = finalStringInitializedToInterned; // OK
    // :: error: (assignment)
    internedStr = finalString2; // error
    @Interned Foo internedFoo = finalFooInitializedToInterned; // OK
    if (arg == finalStringStatic1) {} // OK
    // :: error: (not.interned)
    if (arg == finalStringStatic2) {} // error
    if (arg == HasFields.finalStringStatic3) {} // OK
    // :: error: (not.interned)
    if (arg == HasFields.finalStringStatic4) {} // error
  }

  private @Interned String base;
  static final String BASE_HASHCODE = "hashcode";

  public void foo() {
    if (base == BASE_HASHCODE) {}
  }

  public @Interned String emptyString(boolean b) {
    if (b) {
      return "";
    } else {
      return ("");
    }
  }
}