guava/src/com/google/common/collect/ImmutableMap.java
@CanIgnoreReturnValue
  @Deprecated
  @Override
  public final V put(K k, V v) {
    throw new UnsupportedOperationException();
  }
@CanIgnoreReturnValue
  @Deprecated
  @Override
  public final V put(K k, V v) {
    throw new UnsupportedOperationException();
  }
Guaranteed to throw an exception and leave the map unmodified.

@throws UnsupportedOperationException always
@deprecated Unsupported operation.