public int getClassIndex() {
    if (classIndex < 0) {
      throw new IllegalStateException("class index not yet set");
    }

    return classIndex;
  }
Gets the constant's class index. It is only valid to call this after {@link #setClassIndex} has
been called.

@return {@code >= 0;} the constant's class's constant pool index
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/code/CstInsn.java