Section 9.5 The Object Class
The designers of the Java language went all in on object oriented programming, especially with respect to inheritance. It turns out that all Java classes take part in an inheritance relationship, except for one. Even if we do not declare a custom class as extending another class, Java automatically sets up an inheritance relationship between our class and the special predefined Java class named Object.
We can demonstrate this quickly by adding one more line to Listing 9.4.4, that is, to test if
guilty is also an instanceof Object. Modifying Listing 9.4.3 with the new main(…) method in Listing 9.5.1 and running it, produces the output that follows the listing. An output of three trues tells us that guilty does inherit Object, even though Base did not name Object in the extends. In fact all Java classes extend the Object class. Java sets this up automatically.// Run a test
public static void main(String[] args) {
// Declare a Base type variable and
// assign to a Derived type object
Base guilty = new Derived();
// Check inheritance relationships
System.out.println( guilty instanceof Derived );
System.out.println( guilty instanceof Base );
System.out.println( guilty instanceof Object );
}
instanceof demonstrationjavac Base.java java Base true true true
As we know, inheritance provides derived classes with access to all base class members. So, what do we get from Object? After all, if all classes inherit Object, then any non-private members that we find will be available to all of our classes. A little sleuthing uncovers the OpenJDK source code for the Object class 1 . The source code for two inherited Object class methods are of particular interest. These are
toString() and equals(…). Source code for both is given below in Listing 9.5.2 and Listing 9.5.3.public String toString() {
return getClass().getName() + "@" + Integer.toHexString(hashCode());
}
toString()public boolean equals(Object obj) {
return (this == obj);
}
equals(…)Now we understand why printing an object that does not override
toString() generates that curious output. The default implementation of toString() returns a String constructed by concatenating the class name of an object with "@" and the hexidecimal representation of its unique hash code.Also important to appreciate is that the default
equals() method inherited by all classes simply defers to testing for object identity using the == operator. This explains why the String class overrides equals() to test equality character-by-character (See Subsection 3.3.2). In general we want Strings to be equal when their character sequence is equal, even when they are not identical.github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/Object.java
