Principle

• Strings are poor substitutes for other value types. Such as int, float or BigInteger.
• Strings are poor substitutes for enum types. As discussed in Item 30.
• Strings are poor substitutes for aggregate types. A better approach is simply to write a class to represent the aggregate, often a private static member class (Item 22).

  // Inappropriate use of string as aggregate type

  String compoundKey = className + "#" + i.next();

• Strings are poor substitutes for capabilities(unforgeable key).

  Client-provided string keys are used to identify each thread-local variable:

  // Broken - inappropriate use of string as capability!

  public class ThreadLocal {

  private ThreadLocal() { } // Noninstantiable

  // Sets the current thread's value for the named variable.

  public static void set(String key, Object value);

  // Returns the current thread's value for the named variable.

  public static Object get(String key);

  }

  The root cause: the string keys represent a shared global namespace for thread-local variables.

  public class ThreadLocal {

  private ThreadLocal() { } // Noninstantiable

  public static class Key { // (Capability)

  Key() { }

  }

  // Generates a unique, unforgeable key

  public static Key getKey() {

  return new Key();

  }

  public static void set(Key key, Object value);

  public static Object get(Key key);

  }

  It is a simple, faster and more elegant matter to make this API typesafe by generifying the ThreadLocal class (Item 26):

  public final class ThreadLocal<T> {

  public ThreadLocal() { }

  public void set(T value);

  public T get();

  }

  Summary

  Avoid the natural tendency to represent objects as strings when better data types exist or can be written. Used inappropriately, strings are more cumbersome, less flexible, slower, and more error-prone than other types. Types for which strings are commonly misused include primitive types, enums, and aggregate types.