The Object Class
toString, equals and hashCode, the methods every Java class inherits.
Every class in Java extends java.lang.Object, directly or indirectly, even if you never write extends. That means every object has a few methods inherited from Object. The three you will override most often are toString, equals and hashCode.
toString
toString() returns a text form of the object. It is called automatically when you print an object or join it with a string. The inherited version prints the class name and a hash code, such as Book@1b6d3586, which is rarely useful.
public class Main {
public static void main(String[] args) {
Book b = new Book("Wings of Fire", 1999);
System.out.println(b);
System.out.println("Reading: " + b);
}
}
class Book {
private final String title;
private final int year;
Book(String title, int year) {
this.title = title;
this.year = year;
}
@Override
public String toString() {
return title + " (" + year + ")";
}
}Wings of Fire (1999) Reading: Wings of Fire (1999)
equals: comparing contents
== checks whether two references point to the same object. equals() should check whether two objects are meaningfully equal. The inherited version behaves just like ==, so override it for value-like classes.
import java.util.Objects;
public class Main {
public static void main(String[] args) {
Point a = new Point(2, 3);
Point b = new Point(2, 3);
Point c = a;
System.out.println(a == b); // different objects
System.out.println(a.equals(b)); // same contents
System.out.println(a == c); // same object
}
}
class Point {
private final int x, y;
Point(int x, int y) { this.x = x; this.y = y; }
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Point other)) return false;
return x == other.x && y == other.y;
}
@Override
public int hashCode() {
return Objects.hash(x, y);
}
}false true true
hashCode: the equals contract
Hash-based collections such as HashSet and HashMap first use hashCode() to find the right bucket, then equals() to compare. So Java has a rule: if two objects are equal, they must have the same hash code. Override both together, or neither.
import java.util.HashSet;
import java.util.Objects;
public class Main {
public static void main(String[] args) {
HashSet<Point> visited = new HashSet<>();
visited.add(new Point(1, 1));
visited.add(new Point(1, 1)); // equal to the first, so not added again
visited.add(new Point(2, 5));
System.out.println(visited.size());
System.out.println(visited.contains(new Point(2, 5)));
HashSet<NoHashPoint> broken = new HashSet<>();
broken.add(new NoHashPoint(1, 1));
broken.add(new NoHashPoint(1, 1));
System.out.println(broken.size()); // 2: the set cannot tell they are equal
}
}
class Point {
final int x, y;
Point(int x, int y) { this.x = x; this.y = y; }
@Override public boolean equals(Object o) {
return o instanceof Point p && p.x == x && p.y == y;
}
@Override public int hashCode() { return Objects.hash(x, y); }
}
class NoHashPoint {
final int x, y;
NoHashPoint(int x, int y) { this.x = x; this.y = y; }
@Override public boolean equals(Object o) {
return o instanceof NoHashPoint p && p.x == x && p.y == y;
}
// hashCode() not overridden: equal objects get different hash codes
}2 true 2
getClass
getClass() tells you the real class of an object at run time.
public class Main {
public static void main(String[] args) {
Object[] things = {"text", 42, 3.14, new int[0]};
for (Object t : things) {
System.out.println(t.getClass().getSimpleName());
}
}
}String Integer Double int[]