public int getClassIndex() {
    if (!hasClassIndex()) {
      throw new IllegalStateException("class index not yet set");
    }

    return classIndex;
  }
Gets the class index associated with this instance. The class index may be set only once and
for one of the constants associated with this instance (e.g. a CstMethodRef). It is only valid
to call this after {@link #setClassIndex} has been called.

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