Ready to become a real developer java study -day02

Variable and identifier variable In memory, the variable address is stored and the value of the variable can be assigned. ![[Pasted image 20250417123627.png]] variable has be to assigned with a value before we use this variable. identifier public class java02 { public static void main(String[] args) { //1. Identifier as alphabet characters String username = "lisi"; //2. Identifier as special characters: _ $ String _name = "wangwu"; String $name = "xiaohong"; String user_name$="lisi"; //3. numbers: 0 - 9, but cannot be put at the start String name2 = "lisi"; //4. the identifiers cannot be repeated within one {}-> variables name cannot be repeat with one {} //5. identifiers are case-sensitive in java //6. some identifiers are keywords in java and cannot be used as variable name //String public = "lisi"; //7. camel naming rule String userName = "lisi"; //8. identifiers' length is not limited. } } Data type ![[Pasted image 20250422143909.png]] public class java03 { public static void main(String[] args) { /** * data storage unit: * - bit: the smallest unit for data execution/processing. on(1), off(2) * - byte: the smallest unit for data storage. is a group of 8 bits. * 1 byte = 8 bit * KB, MB, GB, TB... * 1024 Byte = 1KB * 1024KB = 1MB * 1024MB = 1GB * 1024GB = 1TB * * byte : 8 bits * short: 16 bits * int : 32 bits * long: 64 bits * */ //integer data type byte b = 10; short s = 10; int i = 10; long l = 10; //floating point data type /** * float: single precision 6-7 decimal digits * double: double precision 15-16 decimal digits */ float f = 1.05226585231f; //a f/F need to be added at the tail of value double d = 2.055698412698877452; System.out.println(f); //1.0522659 System.out.println(d); //2.0556984126988773 //character type char c = 'c'; //only one character //boolean type boolean bl = true; /** * Type Conversion in Java * 1. widening conversion/implicit casting(small to large type) * byte -> short -> int -> long -> float -> double convert automatically! * * 2. Narrowing conversion/explicit casting (large to small type) * -- may cause data loss and cannot be converted automatically. * -- may cause precision loss or overflow * -- Requires explicit casting using parentheses. * byte

Apr 22, 2025 - 16:45

Ready to become a real developer java study -day02

Variable and identifier variable In memory, the variable address is stored and the value of the variable can be assigned. ![[Pasted image 20250417123627.png]] variable has be to assigned with a value before we use this variable.

identifier

public class java02 {  
    public static void main(String[] args) {  
        //1. Identifier as alphabet characters  
        String username = "lisi";  

        //2. Identifier as special characters: _ $  
        String _name = "wangwu";  
        String $name = "xiaohong";  
        String user_name$="lisi";  

        //3. numbers: 0 - 9, but cannot be put at the start  
        String name2 = "lisi";  

        //4. the identifiers cannot be repeated within one {}-> variables name cannot be repeat with one {}  

        //5. identifiers are case-sensitive in java  
        //6. some identifiers are keywords in java and cannot be used as variable name        //String public = "lisi";  
        //7. camel naming rule        String userName = "lisi";  

        //8. identifiers' length is not limited.  


    }  
}

Data type ![[Pasted image 20250422143909.png]]

public class java03 {  
    public static void main(String[] args) {  
        /**  
         * data storage unit:         * - bit: the smallest unit for data execution/processing. on(1), off(2)         * - byte: the smallest unit for data storage. is a group of 8 bits.         *  1 byte = 8 bit         *  KB, MB, GB, TB...         *  1024 Byte = 1KB         *  1024KB = 1MB         *  1024MB = 1GB         *  1024GB = 1TB         *         * byte : 8 bits         * short: 16 bits         * int : 32 bits         * long: 64 bits         *         */  
        //integer data type  
        byte b = 10;  
        short s = 10;  
        int i = 10;  
        long l = 10;  

        //floating point data type  
        /**  
         * float: single precision 6-7 decimal digits         * double: double precision 15-16 decimal digits         */        float f = 1.05226585231f; //a f/F need to be added at the tail of value  
        double d = 2.055698412698877452;  
        System.out.println(f); //1.0522659  
        System.out.println(d); //2.0556984126988773  


        //character type        char c = 'c'; //only one character  

        //boolean type        boolean bl  = true;  

        /**  
         * Type Conversion in Java         * 1. widening conversion/implicit casting(small to large type)         *  byte -> short -> int -> long -> float -> double convert automatically!         *         * 2. Narrowing conversion/explicit casting (large to small type)         *  -- may cause data loss and cannot be converted automatically.         *  -- may cause precision loss or overflow         *  -- Requires explicit casting using parentheses.         *  byte <- short <- int <- long <- float <- double         */  
        //1. widening conversion  
        byte b2 = 10;  
        int i2 = 100;  
        i2 = b2;  

        //2. narrowing conversion  
        b2 =(byte) i2;  

    }  
}