Java basic programming structure and Java programming structure
Public is the access modifier)
System. out. println (); // wrap automatically after output
System. out. print (); // No line break after output
Note:
1. add // 2 to the front of each line. /* content to be annotated */but not in the annotated code */3. /** content to be annotated */This annotation can automatically generate a document
Data Type:
1. long Integer ending with L 2. the end of float Type F. If no value is added, it is automatically considered as double type 3. calculate the square root of a negative number or the result of 0/0 is NaN (not a number) 4. floating point numbers are not applicable to financial calculations that prohibit rounding errors. Because floating point values are represented by binary systems, they cannot accurately represent 1/10. If you do not need to include any rounding error in numerical calculation, you should use the BigDecimal class. 5. boolean type: true and false, used to determine logical conditions. Integer and boolean values cannot be converted to each other.
Variable:
1. The variable must start with a letter and the variable name is case sensitive. 2. The CamelCase naming rule for variables, but the first letter must be in lower case. 3. Variables without initial values cannot be used.
Constant:
1. final double CM_PER_INCH = 2.54; // The keyword final indicates that this variable can only be assigned once. Once assigned, it cannot be changed.
Relational operators:
1. triplicate operation: conditon? Condition1: condition2 examples: x <y? X: y // return the smaller value of x and y.
Mathematical functions and constants:
For example, to calculate the square root of a value, use the sqrt Method double x = 4; double y = Math. sqrt (x); System. out. println (y); // Add import static java to the top of prints 2.0. lang. math. *; then, Math will not be added before the method name and variable name.
Forced type conversion:
For example, double x = 9.997; int nx = (int) x; // the nx value is 9. If you round the floating point, you can obtain the nearest integer, you need to use Math. round () method: double x = 9.997; int nx = (int) Math. round (x); // the nx value is 10
String:
1. substring: The substring method in the String class can extract a substring from a large String, for example, String greeting = "Hello"; String s = greeting. substring (0th); // creates a string () consisting of "El", indicating that it starts from until 3, but does not include 3, that is, the string length is 3. 2. check whether the string is equal: You can use the equals method: s. equals (t). If the character string s is the same as the string t, true is returned; otherwise, false is returned. you can also: "Hello ". equals ("greeting "). If you check whether the two strings are equal and case-insensitive, you can use the equalsIgnoreCase method: "Hello ". equalsIgnoreCase ("hello") *** Note: you cannot use the = Operator to check whether two strings are equal! ***
Code added by android3.0:
Add android. enableAapt2 = false to gradle. properties.
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