java官方文档汉化 java官方中文文档

语言基础

• 2 变量

• 命名

• 2.1基本数据类型

• 字面值
• 数字中使用下划线

• 2.2 数组

• 练习

• 3 运算符

• 列表
• 赋值、算术和一元运算符
• 等，关系，条件运算符
• 三目运算符
• 对象类型的比较
• 位运算符

• 4.表达式、声明和块

• 表达式
• 练习

• 5. 流程控制

本文接续上一篇：跟着官方文档学Java_jdk1.8_(2)——系列

2 变量

前文说在面向对象的语言中，不用变量这个词。但是Java不讲码德。
所以在Java中，也使用变量。。。（field和varieble都可以用。）

命名

注：Java的官方的规定相对是宽泛的，而在实际的生产环境中，会用更严格的规范。也有助开发。

可以搜一下，网上阿里开发规范。

2.1基本数据类型

Java中有8种基本数据类型：

各类都有默认值

字面值

这里类型各种进制比较多

//整型
// The number 26, in decimal
int decVal = 26;
//  The number 26, in hexadecimal
int hexVal = 0x1a;
// The number 26, in binary
int binVal = 0b11010;

//浮点型
double d1 = 123.4;
// same value as d1, but in scientific notation 这里是d1的科学计数法表示
double d2 = 1.234e2;
float f1  = 123.4f;

值得注意的是，在String 类型的字面值中，有转义字符

String literals:
 \b (backspace), 
 \t (tab),
 \n (line feed), 
 \f (form feed), 
 \r (carriage return),
 \" (double quote),
 \' (single quote), 
 \\ (backslash).

数字中使用下划线

这个功能不是很常用。
用下划线来使数字易读。
规则繁复。各位看官，有兴趣。有用，查一下。

2.2 数组

数组是一种类型的数字的容器。

一旦定义，长度就是固定的。元素可以更改。

上图是一个有10个元素的数组。

可以通过下标来获取每个元素。下标开始的值是0。
以下是一个数组案例：

class ArrayDemo {
    public static void main(String[] args) {
        // declares an array of integers声明一个整数数组
        int[] anArray;

        // allocates memory for 10 integers 分配存储10个元素
        anArray = new int[10];
           
        // initialize first element 第一个元素
        anArray[0] = 100;
        // initialize second element 第二个元素
        anArray[1] = 200;
        // and so forth 依次
        anArray[2] = 300;
        anArray[3] = 400;
        anArray[4] = 500;
        anArray[5] = 600;
        anArray[6] = 700;
        anArray[7] = 800;
        anArray[8] = 900;
        anArray[9] = 1000;

        System.out.println("Element at index 0: "
                           + anArray[0]);
        System.out.println("Element at index 1: "
                           + anArray[1]);
        System.out.println("Element at index 2: "
                           + anArray[2]);
        System.out.println("Element at index 3: "
                           + anArray[3]);
        System.out.println("Element at index 4: "
                           + anArray[4]);
        System.out.println("Element at index 5: "
                           + anArray[5]);
        System.out.println("Element at index 6: "
                           + anArray[6]);
        System.out.println("Element at index 7: "
                           + anArray[7]);
        System.out.println("Element at index 8: "
                           + anArray[8]);
        System.out.println("Element at index 9: "
                           + anArray[9]);
    }
}

打印结果：

除了int，其它的基本数据类型也可以存储在数组中。

String，实例对象也都可以。

二维数组，也用的不多。（从略）

注：数组的长度属性，可以用length来输出。

练习

Questions
1.The term "instance variable" is another name for ___.
2.The term "class variable" is another name for ___.
3.A local variable stores temporary state; it is declared inside a ___.
4.A variable declared within the opening and closing parenthesis of a method signature is called a ____.
5.What are the eight primitive data types supported by the Java programming language?
6.Character strings are represented by the class ___.
7.An ___ is a container object that holds a fixed number of values of a single type.


Exercises
1.Create a small program that defines some fields. Try creating some illegal field names and see what kind of error the compiler produces. Use the naming rules and conventions as a guide.

2.In the program you created in Exercise 1, try leaving the fields uninitialized and print out their values. Try the same with a local variable and see what kind of compiler errors you can produce. Becoming familiar with common compiler errors will make it easier to recognize bugs in your code.

答案：

1. non-static field
2. static field
3. method
4. parameter
5. byte,short,int,long,char,float,double,boolean
6. java.lang.String
7. array

3 运算符

列表

赋值、算术和一元运算符

1. =就是赋值
2. 算术（5种）
   
   如果你学过c/c++
   和它们一样。四则运算，再有一个取余（也称模）。

class ArithmeticDemo {

    public static void main (String[] args) {

        int result = 1 + 2;
        // result is now 3
        System.out.println("1 + 2 = " + result);
        int original_result = result;

        result = result - 1;
        // result is now 2
        System.out.println(original_result + " - 1 = " + result);
        original_result = result;

        result = result * 2;
        // result is now 4
        System.out.println(original_result + " * 2 = " + result);
        original_result = result;

        result = result / 2;
        // result is now 2
        System.out.println(original_result + " / 2 = " + result);
        original_result = result;

        result = result + 8;
        // result is now 10
        System.out.println(original_result + " + 8 = " + result);
        original_result = result;

        result = result % 7;
        // result is now 3
        System.out.println(original_result + " % 7 = " + result);
    }
}

特别地，+也可以作为字符串拼接符号。
举例：

class ConcatDemo {
    public static void main(String[] args){
        String firstString = "This is";
        String secondString = " a concatenated string.";
        String thirdString = firstString+secondString;
        System.out.println(thirdString);
    }
}

输出结果是：“This is a concatenated string.”

3.一元运算符

class UnaryDemo {

    public static void main(String[] args) {

        int result = +1;
        // result is now 1
        System.out.println(result);

        result--;
        // result is now 0
        System.out.println(result);

        result++;
        // result is now 1
        System.out.println(result);

        result = -result;
        // result is now -1
        System.out.println(result);

        boolean success = false;
        // false
        System.out.println(success);
        // true
        System.out.println(!success);
    }
}

比较烦人的是++和--，它们在前和在后，还不太一样。
有些语言就没有这个问题。如Scala.

举个烦人的例子。

class PrePostDemo {
    public static void main(String[] args){
        int i = 3;
        i++;
        // prints 4
        System.out.println(i);
        ++i;			   
        // prints 5
        System.out.println(i);
        // prints 6
        System.out.println(++i);
        // prints 6
        System.out.println(i++);
        // prints 7
        System.out.println(i);
    }
}

等，关系，条件运算符

==      equal to 等于
!=      not equal to 不等于
>       greater than 大于
>=      greater than or equal to 大于或等于
<       less than 小于
<=      less than or equal to 小于或等于

举例

class ComparisonDemo {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        if(value1 == value2)
            System.out.println("value1 == value2");
        if(value1 != value2)
            System.out.println("value1 != value2");
        if(value1 > value2)
            System.out.println("value1 > value2");
        if(value1 < value2)
            System.out.println("value1 < value2");
        if(value1 <= value2)
            System.out.println("value1 <= value2");
    }
}

&& Conditional-AND 与
|| Conditional-OR 或

也可将!算作逻辑运行符非

class ConditionalDemo1 {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        if((value1 == 1) && (value2 == 2))
            System.out.println("value1 is 1 AND value2 is 2");
        if((value1 == 1) || (value2 == 1))
            System.out.println("value1 is 1 OR value2 is 1");
    }
}

三目运算符

也算在逻辑运算符里。
直接举例

class ConditionalDemo2 {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        int result;
        boolean someCondition = true;
        result = someCondition ? value1 : value2;

        System.out.println(result);
    }
}

对象类型的比较

关键字instanceof

class InstanceofDemo {
    public static void main(String[] args) {

        Parent obj1 = new Parent();
        Parent obj2 = new Child();

        System.out.println("obj1 instanceof Parent: "
            + (obj1 instanceof Parent));
        System.out.println("obj1 instanceof Child: "
            + (obj1 instanceof Child));
        System.out.println("obj1 instanceof MyInterface: "
            + (obj1 instanceof MyInterface));
        System.out.println("obj2 instanceof Parent: "
            + (obj2 instanceof Parent));
        System.out.println("obj2 instanceof Child: "
            + (obj2 instanceof Child));
        System.out.println("obj2 instanceof MyInterface: "
            + (obj2 instanceof MyInterface));
    }
}

class Parent {}
class Child extends Parent implements MyInterface {}
interface MyInterface {}

位运算符

是基于底层，二进制数计算。

class BitDemo {
    public static void main(String[] args) {
        int bitmask = 0x000F;
        int val = 0x2222;
        // prints "2"
        System.out.println(val & bitmask);
    }
}

一般开发者，好像不常用？？？

Questions
1. Consider the following code snippet.
arrayOfInts[j] > arrayOfInts[j+1]
Which operators does the code contain?

2.Consider the following code snippet.
int i = 10;
int n = i++%5;
What are the values of i and n after the code is executed?
What are the final values of i and n if instead of using the postfix increment operator (i++), you use the prefix version (++i))?

3.To invert the value of a boolean, which operator would you use?

4.Which operator is used to compare two values, = or == ?

5.Explain the following code sample: result = someCondition ? value1 : value2;


Exercises
1.Change the following program to use compound assignments:
class ArithmeticDemo {

     public static void main (String[] args){
          
          int result = 1 + 2; // result is now 3
          System.out.println(result);

          result = result - 1; // result is now 2
          System.out.println(result);

          result = result * 2; // result is now 4
          System.out.println(result);

          result = result / 2; // result is now 2
          System.out.println(result);

          result = result + 8; // result is now 10
          result = result % 7; // result is now 3
          System.out.println(result);
     }
}

2.In the following program, explain why the value "6" is printed twice in a row:
class PrePostDemo {
    public static void main(String[] args){
        int i = 3;
        i++;
        System.out.println(i);    // "4"
        ++i;                     
        System.out.println(i);    // "5"
        System.out.println(++i);  // "6"
        System.out.println(i++);  // "6"
        System.out.println(i);    // "7"
    }
}

Q：
1. >大于号 +加号
2.i = 11; n = 0;   i = 11; n = 1;
3.!
4.==
5.如果result = someCondition, output value1; otherwise, assign the value2

4.表达式、声明和块

表达式

int cadence = 0;
anArray[0] = 100;
System.out.println("Element 1 at index 0: " + anArray[0]);

int result = 1 + 2; // result is now 3
if (value1 == value2) 
    System.out.println("value1 == value2");

如上，是表达式的例子。
总的来说，就是要清晰。

声明和语句块，略。比较简单。

练习

Questions
Operators may be used in building ___, which compute values.
Expressions are the core components of ___.
Statements may be grouped into ___.
The following code snippet is an example of a ___ expression.
 1 * 2 * 3
Statements are roughly equivalent to sentences in natural languages, but instead of ending with a period, a statement ends with a ___.
A block is a group of zero or more statements between balanced ___ and can be used anywhere a single statement is allowed.
Exercises
Identify the following kinds of expression statements:

aValue = 8933.234;
aValue++;
System.out.println("Hello World!");
Bicycle myBike = new Bicycle();

5. 流程控制