Common Java classes: Object class
summary
- It is a super class and base class, which is located at the top of the inheritance number. It is the direct or indirect parent of all classes
- As long as any class does not declare extends to inherit a class, it will inherit the Object class by default. Otherwise, it will inherit the Object class indirectly
- The methods defined in the Object class are the methods of all objects, so subclasses can use all methods of Object
All methods:
| Modifiers and types | Method and description |
|---|---|
| protected Object | clone() creates and returns a copy of this object. |
| boolean | equals(Object obj) indicates whether some other object is equal to this. |
| protected void | finalize() when the garbage collector determines that there is no longer a reference to the object, the garbage collector calls the object on the object. |
| Class<?> | getClass() returns the runtime class of this Object. |
| int | hashCode() returns the hash code value of the object. |
| void | notify() wakes up a single thread waiting for the object monitor. |
| void | notifyAll() wakes up all threads waiting for the object monitor. |
| String | toString() returns the string representation of the object. |
| void | wait() causes the current thread to wait until another thread calls the notify() method or notifyAll() method of the object. |
| void | wait(long timeout) causes the current thread to wait until another thread calls the notify() method or the notifyAll() method of the object, or the specified time has passed. |
| void | wait(long timeout, int nanos) causes the current thread to wait until another thread calls the notify() method or notifyAll() method of the object, or some other thread interrupts the current thread, or a certain amount of real-time time time. |
Demonstration class:
public class Student {
private String name;
private int age;
public Student(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}
getClass() method
-
Type: class <? >
-
Returns the actual object type stored in the reference
-
Function: it is usually used to judge whether the actual storage object types in two references are consistent
-
Example:
public class TestStudent { public static void main(String[] args) { Student s1 = new Student("lisa",20); Student s2 = new Student("kirot", 21); //Judge whether s1 and s2 are of the same type: getClass(); Class class1 = s1.getClass(); Class class2 = s2.getClass(); System.out.println(class1 == class2 ? "s1 and s2 Belong to the same type" : "s1 and s2 Not of the same type"); } }output
s1 and s2 Belong to the same type
hashCode() method
-
Type: int
-
Returns the hash code value of the object
-
Hash value a numeric value of type int calculated using the hash algorithm based on the address or string or number of the object
-
Generally, the same object returns the same hash code value
-
Example:
public class TestStudent { public static void main(String[] args) { Student s1 = new Student("lisa",20); Student s2 = new Student("kirot", 21); //Returns the hash code values of s1 and s2. The hash codes returned by different objects are different System.out.println(s1.hashCode()); System.out.println(s2.hashCode()); Student s3 = s1; //s1 address is assigned to s3, so their hash code values are the same System.out.println(s3.hashCode()); } }output
2129789493 668386784 2129789493
toString() method
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Type: String
-
Returns the string representation of the object
-
Example:
public class TestStudent { public static void main(String[] args) { Student s1 = new Student("lisa",20); Student s2 = new Student("kirot", 21); //Returns the string representation of the object: class name + @ + hexadecimal hash code value System.out.println(s1.toString()); System.out.println(s2.toString()); } }output
Student@7ef20235 Student@27d6c5e0
-
Extension: you can also override the toString() method in the class to return your own custom string
-
Example:
Override Student class
public class Student { private String name; private int age; public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "Student{" + "name='" + name + '\'' + ", age=" + age + '}'; } }public class TestStudent { public static void main(String[] args) { Student s1 = new Student("lisa",20); Student s2 = new Student("kirot", 21); //Returns the string representation of the object System.out.println(s1.toString()); System.out.println(s2.toString()); } }output
Student{name='lisa', age=20} Student{name='kirot', age=21}
equals() method
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Type: boolean
-
The default implementation is (this == obj). Compare whether the addresses of two objects are the same
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You can overwrite and compare whether the contents of the two objects are the same
-
Example:
public class TestStudent { public static void main(String[] args) { Student s1 = new Student("lisa",20); Student s2 = new Student("kirot", 21); //equals(); Judge whether two objects are equal System.out.println(s1.equals(s2)); Student s3 = s1; //s1 address assigned to s3 System.out.println(s1.equals(s3)); Student s4 = new Student("w",18); Student s5 = new Student("w",18); System.out.println(s4.equals(s5)); //The addresses of s4 and s5 are different } }output
false true false
Override steps for the equals() method
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Compare whether two references point to the same object
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Judge whether obj is null
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Judge whether the actual object types pointed to by the two references are consistent
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Cast type
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Compare whether the attribute values are the same in turn
-
Examples
Override the equals() method
public class Student { private String name; private int age; public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public boolean equals(Object obj) { //1. Judge whether two objects are the same reference if (this == obj) { return true; } //2. Judge whether obj is null if (obj == null) { return false; } //3. Judge whether it is the same type if (obj instanceof Student) { //instanceof determines whether an object is of a certain type //4. Cast type Student s = (Student) obj; //5. Compare attributes if (this.name.equals(s.getName()) && this.age == s.getAge()) { return true; } } return false; } }public static void main(String[] args) { Student s1 = new Student("lisa",20); Student s2 = new Student("kirot", 21); //equals(); Judge whether two objects are equal System.out.println(s1.equals(s2)); Student s3 = s1; //s1 address assigned to s3 System.out.println(s1.equals(s3)); Student s4 = new Student("w",18); Student s5 = new Student("w",18); System.out.println(s4.equals(s5)); }output
false true true
finalize() method (deprecated)
- When the object is determined to be a garbage object, the JVM will automatically call this method to mark the garbage object and enter the collection queue
- Garbage object: garbage object when there is no valid reference to this object
- Garbage collection: GC destroys garbage objects to free up data storage space
- Automatic recycling mechanism: the JVM runs out of memory and recycles all garbage objects at one time
- Manual recycling mechanism: use system gc(); Notify the JVM to perform garbage collection
clone() method
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Create and return a copy of this object
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clone() is a shallow copy. The object referenced by the attribute in the object will only copy the reference address without reallocating the memory of the referenced object. The corresponding deep copy will recreate the referenced object
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Because the Object itself does not implement the clonable interface, CloneNotSupportedException will occur if the clone method is not rewritten and called
-
Example:
public class Student implements Cloneable{ private String name; private int age; public Student(String name, int age) { this.name = name; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public static void main(String[] args) { Student s1 = new Student("dt", 20); System.out.println(s1.getName()+"\t"+s1.getAge()); try { //Create a copy of s1 Student s2 = (Student) s1.clone(); System.out.println(s2.getName()+"\t"+s2.getAge()); } catch (CloneNotSupportedException e) { e.printStackTrace(); } } }output
dt 20 dt 20