public void setClassIndex(int index) {
    if (index < 0) {
      throw new IllegalArgumentException("index < 0");
    }

    if (hasClassIndex()) {
      throw new IllegalStateException("class index already set");
    }

    this.classIndex = index;
  }
Sets the class index associated with this instruction. This is the constant pool index for the
class referred to by this instruction. Only reference constants have a class, so it is only on
instances with reference constants that this method should ever be called. It is only valid to
call this method once per instance.

@param index {@code index >= 0;} the constant pool index of the class
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/code/MultiCstInsn.java