1.String class
-
String is a constant and cannot be changed after creation
-
String literals are stored in the string pool and can be shared
-
String s = "hello"； Generate an object and store it in the string pool
-
String s = new String("hello"); Two objects are generated, one in heap and one in pool
common method
-
public int length(): returns the length of the string
-
public char charAt(int index): obtain characters according to the offline table
-
public boolean contains(String str): judge whether the current string contains str
-
public char[] toCharArray(): convert string to array
-
public int indexOf(String str): find the subscript that appears for the first time in str. if it exists, the subscript will be returned; If it does not exist, - 1 is returned;
-
public int lastindexOf(String str): find the index of the last occurrence of the string in the current string
-
public String trim(): remove the spaces before and after the string
-
public String toUpperCase(): convert lowercase to uppercase
-
public boolean endWith(String str): determines whether the string ends with str
-
public String replace(char oldChar,char new Char); Replace old string with new string
-
public String[] split(String str): split according to str
2.Object class
-
Superclass, base class, direct or indirect parent class of all classes, located at the top of the inheritance tree
-
For any class, if extensions is not written to show that it inherits a class, it inherits the Object class directly by default, otherwise it inherits indirectly
-
The methods defined in the Object class are the methods that all objects have
-
The Object type can store any Object
-
As a parameter, any object can be accepted
-
As a return value, any object can be returned
-
1.getClass() method
-
public final Class<?> getClass(){}
-
Returns the actual object type stored in the reference
-
Application: it is usually used to judge whether the actual storage object types in two references are consistent
public class Student {
private String name;
private int 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 Student() {
}
public Student(String name, int age) {
this.name = name;
this.age = age;
}
}
p
ublic class TestStudent {
public static void main(String[] args) {
Student s1 = new Student("aaa",20);
Student s2 = new Student("bbb",22);
//Judge whether s1 and s2 are of the same type (getClass method)
Class class1 = s1.getClass();
Class class2= s2.getClass();
if(class1==class2){
System.out.println("s1 and s2 Belong to the same type");
}else {
System.out.println("s1 and s2 Not of the same type");
}
}//The result output belongs to the same class
}
2.hashCode() method
-
public int hashCode(){}
-
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
//hashCode method System.out.println(s1.hashCode());//460141958 System.out.println(s2.hashCode());//1163157884 Student s3 = s1; System.out.println(s3.hashCode());//460141958 is equal to s1
3.toString() method
-
public String toString(){}
-
Returns a string representation of the object
-
This method can be overridden according to program requirements, such as displaying the attribute values of the object
public String toString() {
return getClass().getName() + "@" + Integer.toHexString(hashCode());
}4.equals() method
-
public boolean equals(Object obj){}
-
The default implementation is (this == obj). Compare whether the addresses of two objects are the same
-
You can overwrite and compare whether the contents of the two objects are the same
//The equals method determines whether two objects are equal System.out.println(s1.equals(s2));//false
public boolean equals(Object obj) {
return (this == obj);
}
5.finalize() method
-
When the object is determined to be a garbage object, the JVM automatically calls 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 collection mechanism: use System.gc(): notify the JVM to perform garbage collection
Type conversion, packing and unpacking
-
Six common methods are provided in the Number parent class
-
parseXXX() static method
public class demo1 {
public static void main(String[] args) {
//Type conversion: the process of converting a basic type into an application type
int num1 = 18 ;
//Reference the Inter class to create an object
Integer integer = new Integer(num1);
Integer integer1 = Integer.valueOf(num1);
System.out.println("Packing");
System.out.println(integer);//18
System.out.println(integer1);//18
//Type conversion: unpack and convert reference type to basic type
Integer integer2 = new Integer(100);
int num2=integer2.intValue();
System.out.println("Unpacking");
System.out.println(num2);//100
//After jdk1.5, automatic packing and unpacking are provided
int age = 30;
//Automatic packing
Integer integer3 = age;
System.out.println(integer3);//30
//Automatic unpacking
int age2=integer3;
System.out.println(age2);//30
System.out.println("---------------------------");
//Conversion between basic type and string
//1 convert basic type to string
int n1 =100 ;
//1.1 use + sign
String s1 = n1+"";
System.out.println(s1.getClass().getName());
//1.2 using toString() method in Integer
String s2 = Integer.toString(n1);
System.out.println(s1);//100
System.out.println(s2);//100
//2. Convert string to basic type
String str = "150" ;
//Use Integer.parseXXX();
int n2=Integer.parseInt(str);
System.out.println(n2);//150
//Convert boolean string form to basic type "true" -- > true
String str2 = "true";
boolean b1 = Boolean.parseBoolean(str2);
System.out.println(b1);//true
}
}