一.Android java基础知识

第一个java程序Hello Worid：

public class Hello {public static void main(String args[]) {System.out.println("Hello, world!");}
}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Hello.java
root@ubuntu:/home/topeet/guyilian# java Hello
Hello, world!

循环打印的例子：

public class Hello {public static void main(String args[]) {int i = 0;for (i = 0; i < 3; i++) {System.out.println("Hello, world!");}}
}

运行结果： 

root@ubuntu:/home/topeet/guyilian# javac Hello.java 
root@ubuntu:/home/topeet/guyilian# java Hello
Hello, world!
Hello, world!
Hello, world!

java与C语言的数据类型对比，java中无指针类型的数据：

public class Var {public static void main(String args[]) {int   a = 3;float f = (float)3.14;float f2 = 3.14f;int i = 4;short s = 4;short s2 = (short)40000;//s = i;s = (short)(s + 1);  s = (short)(s + s2);/* Java has no pointer *///int* p = malloc(10*sizeof(int));int p[] = new int[10];int p2[] = {1,2,4}; /* static alloc *///char str[100];char str[] = new char[100];//char str2[] = "abc";String str2 = "abc";p = null;p2 = null;str = null;str2 = null;}
}

与C语言相比，Java的函数可以进行重载的操作，对函数的个数以及函数参数的类型也能够进行重载

public class Hello {public static void main(String args[]) {System.out.println(add(1,2));System.out.println(add(1,2, 3));System.out.println(add(1.0f, 2.0f));}public static int add (int x, int y) {return x + y;	}public static int add (int x, int y, int z) {return x + y + z;	}public static float add (float x, float y) {return x + y;	}}

运行结果：

root@ubuntu:/home/topeet/guyilian# javac Hello.java 
root@ubuntu:/home/topeet/guyilian# java Hello
3
6
3.0

函数传递参数，如果要修改传递的参数要使用指针，但是在java里面用的是数组的地址。

public class Hello {public static void main(String args[]) {int x = 1;fun (x);int p[] = new int[1];p[0] = 123;System.out.println("Before fun2: "+p[0]);fun2(p);System.out.println("After fun2: "+p[0]);System.out.println(x);}public static void fun(int x) {x = 100;}		public static void fun2(int[] p) {p[0] = 200;}		}

运行结果：

root@ubuntu:/home/topeet/guyilian# java Hello
Before fun2: 123
After fun2: 200
1