Day11—API—Object

API

summary

Application Programming Interface

Interface oriented programming - interface documentation

API development manual

On the official website

Object

summary

• Package: Java lang ； Default auto guided package

• Class Object

• Class Object is the root class of the class hierarchy

• Each class uses Object as the superclass (Object is the top-level parent of all classes)

• Each class inherits the Object class

Construction method

Object() - parameterless construction

method

Clone

❤protected Object clone()

copy object

1. protected can only be used in this class, the same package class and subclasses

2. You can override the clone method class in OBject through subclasses to solve the problem of permission modifiers

3. Clonable interface: mark the interface (there is no abstract method in the interface)

If it is not implemented, an exception will be thrown, CloneNotSupportedException

1. The return value type is Object, which reflects the upward modeling

2. Superficial replication deep replication

code

Teacher teacher1 =newTeacher("Teacher 1",18);
Teacher teacher2 =newTeacher("Teacher 2",18);
Student student1 =newStudent("gakki",18,100, teacher1);
Student student2 =newStudent("Yuna Ito ",19,100, teacher2);
Student[] students ={student1,student2};

//First output the name of the teacher of the original array object
System.out.println(students[0].getTeacher().getName());
Student[] clone = students.clone();
//Modify reference
clone[0].getTeacher().setName("Modified");
System.out.println(clone[0].getTeacher().getName());
//Check whether the address has been modified
System.out.println(students[0].getTeacher().getName());
System.out.println("students: "+students);
System.out.println("clone: "+clone);
System.out.println("____________________________________________________________________________");
System.out.println(Arrays.toString(students));
System.out.println(Arrays.toString(clone));
System.out.println("____________________________________________________________________________");
System.out.println(students[0].getTeacher()+" "+students[1].getTeacher());
System.out.println(clone[0].getTeacher()+" "+clone[1].getTeacher());

Shallow replication

Deep replication

• protected void finalize() - this method is called by the garbage collector and cannot recycle objects; Notify the garbage collector that the garbage is generated by garbage, but the collection time is uncertain

• public final Class<?> GetClass () - returns the runtime class of this OBject

• int hashCode() - returns the hash code value of the object

Summary: different objects have different hash code values. If the hash code values are the same, the same object should be executed

toString

• ❤ String toString -- returns the string representation of the queue

Object of toString Source code
public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}
//java.lang.Object@28d93b30

//Object toString source code
//The print result is the same as that of Object
System.out.println(new Student());

//println source code
public void println(Object x) {
    String s = String.valueOf(x);
    synchronized (this) {
        print(s);
        newLine();
    }
}

//String.valueOf source code
public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

use

The direct output Object calls the toString method of the Object, and most classes inherit the toString method from Object. The default is to print the address value; If you want to output the string spliced by the attributes of the Object, you can override the toString method; When the Object is output again, the toString method after subclass rewriting is called;

toString method can be generated directly through IDE

euqals

• ❤ boolean equals(Object obj) - compares whether the current object and the parameter object are equal

equals()Source code
public boolean equals(Object obj) {
    return (this == obj);
}

Conclusion: the equals method in Object is also used to compare address values

The ratio of two variables is called equal

1. ==

Basic data type: compare whether two data values are equal

Reference data type: compare whether the address values of two variables are equal

1. Override the equals method

@Override
public boolean equals(Object o) {
  //1. Judge whether it is the same object
  if (this == o) return true;
  //2. Judge whether the parameter is null and whether the two are of the same type
  if (o == null || getClass() != o.getClass()) return false;
  //3. Convert parameter type to current type
  Student student = (Student) o;
  //4. Compare the specific attributes of objects in turn
  if (age != student.age) return false;
  if (score != student.score) return false;
  if (!name.equals(student.name)) return false;
  returnteacher.equals(student.teacher);
  }
  //It is best to override the hasCode method
  @Override
public int hashCode() {
  int result = name.hashCode();
  result = 31 * result + age;
  result = 31 * result + score;
  result = 31 * result + teacher.hashCode();
  return result;
}