TypeToken.java

/**
 * Represents a parameterized type. Unfortunately necessary because the JVM doesn't support
 * reified generics in all cases.
 */
@SuppressWarnings("unchecked")
public class TypeToken<T> {

  /**
   * The type represented by this type token.
   */
  public final Class<T> type;
  
  /**
   * The type parameters to the type represented by this type token.
   */
  public final TypeToken<?>[] parameters;

  /**
   * Creates a new type token for the given type, parameterized with the given type parameters.
   */
  public TypeToken(Class<T> type, TypeToken<?>... parameters) {
    this.type = type;
    this.parameters = parameters;
  }
  
  /**
   * Attempts to create a type token by using the JVM's limited generic reification capabilities.
   * This constructor must be invoked from a subtype to work!
   */
  protected TypeToken() {
    Type thisType = getClass().getGenericSuperclass();
    try {
      Type tp = ((ParameterizedType)thisType).getActualTypeArguments()[0];
      if (tp instanceof ParameterizedType) {
        ParameterizedType ptp = (ParameterizedType)tp;
        this.type = (Class<T>)ptp.getRawType();
        this.parameters = Arrays.stream(ptp.getActualTypeArguments())
            .map(TypeToken::new).toArray(TypeToken[]::new);
      } else {
        this.type = (Class<T>)tp;
        this.parameters = new TypeToken[0];
      }
    } catch (Exception e) {
      throw new UnsupportedOperationException("Could not compute parameterized type for " + thisType, e);
    }
  }
  
  /**
   * Attempts to create a type token from a reified generic type, assuming all its parameters are also reified.
   */
  private TypeToken(Type tp) {
    try {
      if (tp instanceof ParameterizedType) {
        ParameterizedType ptp = (ParameterizedType)tp;
        this.type = (Class<T>)ptp.getRawType();
        this.parameters = Arrays.stream(ptp.getActualTypeArguments())
            .map(TypeToken::new).toArray(TypeToken[]::new);
      } else {
        this.type = (Class<T>)tp;
        this.parameters = new TypeToken[0];
      }
    } catch (Exception e) {
      throw new UnsupportedOperationException("Could not compute parameterized type for " + tp, e);
    }
  }
  
  @Override
  public String toString() {
    StringBuilder buf = new StringBuilder(type.getSimpleName());
    if (parameters.length > 0) {
      buf.append("<").append(parameters[0].toString());
      for (int i = 1; i < parameters.length; i++) {
        buf.append(", ").append(parameters[i].toString());
      }
      buf.append(">");
    }
    return buf.toString();
  }

}