Security analysis and examples of Java data types and variables

1. Identifiers

It is stipulated in Java that identifiers can only contain: letters, numbers, underscores and $ symbols , but cannot start with a number, and is strictly case-sensitive.

Reference standards:
1. Class name: big camel case, the first letter of each word is capitalized (HelloWorld)
2. Method name: small camel case, capitalize the first letter of each word starting from the second word (helloWorld)
3. Variable name: small camel case

The illegal identifiers are as follows (Example):

• 1.public(keyword)

• 2.520shy(beginning with number)

• 3.zhao.d (Unfamiliar symbols appear)

2. Data type

Both integers and floating point numbers in java It is strictly with sign bit

There are 8 basic data types in java

as follows (example ):

Everyone should pay special attention to:
1. In java, the character type occupies 2 bytes, because The C language uses ASCII encoding, and the Java language uses unicode encoding.
2. No matter in x64 or x86 environment, int occupies 4 bytes and long occupies 8 bytes.

3. Variables

Because Java is a very safe language, when we get started, we will make various small mistakes. Let us explore it together. Let’s take a look.

1. No initialization

 public static void main(String[] args) {
        int a;
        System.out.println(a);
    }

In java, local variables are not initialized, and the system will compile errors.

Solution:
1.Initialization
2.Assignment

public static void main(String[] args) {
        int a;
        a=10;//1.赋值
        System.out.println(a);
        int b=10;//2.初始化
        System.out.println(b);
    }

2.Assignment exceeds Range

public static void main(String[] args) {
        int a=2147483648;
        System.out.println(a);
    }

Here we discuss the range of integers: because java data has a sign bit.

Here we use Java’s wrapper class for calculation:

We can I see that the value assigned in the top program exceeds the range of Int, so an error will be reported during compilation.

3. The use of long and float

The default integer type in java It is int type, and the floating point type defaults to double

has the following provisions:

• 1. When defining the float type, add an f

• # after the data. 2. When defining the long type, add an L

public static void main(String[] args) {
        float a=3.5f;
        long b=10L;
    }

after the data. 4. Use of division

public static void main(String[] args) {
        int a=3;
        int b=2;
        System.out.println(a/b);
    }

Will 1.5 be output here?

In Java, int values ​​can only store the integer part, regardless of the decimal number, discard it directly

Solution ：

  public static void main(String[] args) {
        int a=3;
        int b=2;
        System.out.println(a*1.0/b);
        double c=3.0;
        double d=2.0;
        System.out.println(c/d);
    }

5. The precision problem of floating point numbers

public static void main(String[] args) {
        double ret=1.1;
        System.out.println(ret*ret);
    }

Because floating point numbers are Storage issues in memory. Double type memory complies with the IEEE 754 standard. Because there will be a certain precision error during storage, the floating point number is not an exact value, but an approximate value.

6 .Use of boolean type variables

public static void main(String[] args) {
        double ret=1.1;
        System.out.println(ret*ret);
    }

There is no saying in Java that 0 means false and 1 means true.
There are only two types of boolean type variables Value, true means true, false means false

7. Type promotion

Case 1:

public static void main(String[] args) {
        int a=2;
        long b=3L;
        int c=a+b;
    }

int与long进行计算时，会把int转换成long，所以存放给int时会报错.

情况2：

public static void main(String[] args) {
        short a=10;
        short b=20;
        short c=a+b;
    }

原因如下：a和b都是short，在运算过程中会把a,b都提升为Int在计算.
CPU通常是按照4个字节从内存中读写数据，为了实现方便，所以低于4字节的类型，会先提升为Int，在计算.

四、初始字符串

在java中引入了一种新的数据类型：字符串类型.

public static void main(String[] args) {
        String s1="woyao";
        String s2="jindachang";
        System.out.println(s1+s2);
    }

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