Understanding Java Variables: Types, Naming Rules, and Type Conversions

In Java, a variable is a mutable storage location, analogous to a plate that can hold different dishes at different times; the article begins with this analogy and shows a simple example of storing the string "清炒黄瓜" in a variable.

The standard syntax is DataType variableName = value;, and three essential aspects are highlighted: data types, variable names, and values.

Data types are divided into primitive (value) types—byte, short, int, long, float, double, char, boolean—and reference types, with String being the most commonly used reference type.

Multiple types exist to represent different kinds of data, just as various dishes require different utensils.

Variable naming rules require the name to start with a letter or underscore, may contain letters, digits, underscores, or the dollar sign, must not be a Java keyword, and should be meaningful (e.g., name instead of n1).

Values must be compatible with the variable's type; an example shows how to store an employee’s information using appropriate types:

public static void main(String[] args){
    String name = "李大爷"; // 姓名
    int salary = 8600; // 薪水
    char sex = '女'; // 性别
    double height = 1.83; // 身高
    boolean isDangYuan = false; // 是否党员
}

Automatic (widening) type conversion occurs when a smaller type is assigned to a larger type, as demonstrated:

public static void main(String[] args){
    char c = '哈'; // 最小
    int i = c;      // 稍大
    float f = i;    // 较大
    double d = f;   // 最大
}

Explicit (narrowing) casting is required when assigning a larger type to a smaller one:

public static void main(String[] args){
    double d = 86.5;
    float f = (float)d;
    int i = (int)d;
    char c = (char)i;
}

Conversions between strings and other types are also covered, showing the most common methods:

public static void main(String[] args){
    String s = "10";
    int i = Integer.parseInt(s); // string to int
    String s1 = String.valueOf(i);
    String s2 = Integer.toString(i);
    String s3 = "" + i; // int to string via concatenation
}

The article concludes by encouraging readers to consult the Java API documentation for deeper details.