Java Study Notes (Introduction)_Program flow control structure and methods

Program flow control structures and methods
Program flow control structures are divided into: sequence, selection, loop and exception handling structures. Statements are the basic building blocks of programs. In Java, there are simple statements and matching statements. A simple statement is a line of code, for example, privateint3=3; a compound statement is a combination of simple statements, such as a method, etc. Generally speaking, the execution flow of statements is carried out in order, but when encountering some special conditions, such as loops, the statements will be carried out according to the process control structure.
(1) Selection structure
The selection structure is used to implement different operations based on different conditions. It provides a mechanism that allows the program to run corresponding statements based on corresponding conditions. There are two forms of Java language implementation selection structure: one is an if-else statement with two-way branch selection, and the other is a switch statement with multi-branch selection. Selecting statements requires the use of logic, but it is relatively simple, such as the true or false of a proposition, whether it is true or not, etc. Logical propositions are used to represent logical expressions and serve as logical conditions for two-way branch or multi-way branch structures.
Obviously, we are more concerned about the writing of conditions, which generally include: relational expressions, logical expressions and conditional operation expressions.
①Relational expression: An expression that connects two expressions using relational operators. Calculate the values ​​of two expressions of the same type and then compare them. The result is: true (true) or false (false). For example:
x%2==0;
x+y>=0;
②Logical expression: The operand is a logical value and the expression of the expression connected with logical symbols becomes a logical expression, and its value is still logical value. For example:
x>6&&y<3;
x>6||y>8;
y%4==0&&y%100!=0&&y%400==0//y is a leap year condition
③Conditional operation expression: by Expressions connected by the ternary operator, the syntax format is: (logical expression)? (expression 1): (expression 2). When the value of the logical expression is true, the value of expression1 is returned, otherwise, the value of expression2 is returned.
(2)if-else statement
The general if-else statement is like this,

if(逻辑表达式){或if(逻辑表达式)语句1； 
语句1；[else语句2；] 
}else{ 
语句2； 
}

The if statement is a statement specially used to implement the selection structure. It decides to run the two operations based on the true or false of the logical condition. A sort of. For example: the conditions for a leap year are: a year that is divisible by 4 but not divisible by 100, or is divisible by 400. Therefore, the judgment of leap year can be expressed by a logical expression.
Let’s determine whether 2012 is a leap year:

publicclassIsLeapYear{ 
publicstaticvoidmain(Stringargs[]){ 
intyear=2012; 
booleanleapYear=(year%4==0&&year%100!=0||year%400==0); 
if(leapYear){ 
System.out.println(year+"是闰年"); 
}else{ 
System.out.println(year+"不是闰年"); 
} 
} 
}

Nesting of if-else statements:
Statement 1 or statement 2 in the if-else statement can also be an if-else statement, thus forming Nesting of if-else statements. The most commonly used one is the multi-selection structure nested with elseif statements:

if()语句1 
elseif(逻辑表达式)语句2 
........ 
elseif(逻辑表达式)语句n 
else语句n+1

When the program is running, it will judge the logical conditions from top to bottom. Once a certain logical condition is met (that is, the value of the Boolean expression is true), the corresponding statement, then it will no longer judge other conditions, go directly to the structure exit, and run the subsequent statements of the if statement. Of course, in this multi-choice structure, it is easier to confuse the matching relationship between if and else. The Java language stipulates that else is always paired with the if closest to it. If necessary, you can use curly braces {} to change the pairing relationship. In fact, we often do this.

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