1.UUID(生成时间戳)

import java.util.UUID;

public class UUIDTest {

public static void main(String[] args) {

UUID uuid = UUID.randomUUID();

System.out.println(uuid.toString());

}

}

运行效果: 生成一个唯一识别码

2.BigInteger、BigDecimal(生成一个很大的数)。

使用场合: 一般的开发之中使用的数据已经都包含在给定的数据的范围之中。但是在很多时候有一些数据真的很大，并且

超过了预期的范围。在原始程序之中面对这样的问题可以使用字符串来解决

import java.math.BigInteger;

public class BigMath {

public static void main(String[] args) {

BigInteger bigA = new BigInteger("845465465465465464654");

BigInteger bigB = new BigInteger("546546546546546546");

System.out.println("加法计算 : " + bigA.add(bigB));

System.out.println("减法计算 : " + bigA.subtract(bigB));

System.out.println("乘法计算 : " + bigA.multiply(bigB));

System.out.println("除法计算 : " + bigA.divide(bigB));

BigInteger result [] = bigA.divideAndRemainder(bigB);

System.out.println("商 = " + result[0] + ",余数 = " + result[1]);

}

}

运行效果:

3.Arrays类(对数组进行操作)

import java.util.Arrays;

public class ArraysTest {

public static void main(String[] args) {

// TODO Auto-generated method stub

int dataA [] = new int [] {1,2,3,4,5,6};

int dataB [] = new int [] {1,2,3,4,5,6};

System.out.println(Arrays.toString(dataA));

System.out.println(Arrays.equals(dataA, dataB));

System.out.println(Arrays.binarySearch(dataA,55));

}

}

运行效果:

4.System类(用来计算程序花费时间)

public static long currentTimeMillis()

public class SystemTest {

public static void main(String[] args) {

// TODO Auto-generated method stub

String str = "";

long start = System.currentTimeMillis();

for(int i = 0 ; i < 10000; i ++) {

str += i;

}

long end = System.currentTimeMillis();

System.out.println("花费时间: " + (double)(end-start)/1000 + "s");

}

}

运行效果:

5.Runtime类(观察内存): 每个JVM进程之中都会有一个Runtime类的对象

public long freeMemory() 取得当前空余内存空间大小

public long totalMemory() 取得当前可以使用的总空间

public long maxMemory()    取得最大的可用空间

public void gc() 执行垃圾回收处理

public class RuntimeTest {

public static void main(String[] args) {

// TODO Auto-generated method stub

Runtime run = Runtime.getRuntime();

System.out.println("max = " + byteToM(run.maxMemory()));

System.out.println("total = " + byteToM(run.totalMemory()));

System.out.println("free = " + byteToM(run.freeMemory()));

for(int i = 0; i < 2222; i++) {

String str = i + "";

}

run.gc();

System.out.println("max = " + byteToM(run.maxMemory()));

System.out.println("total = " + byteToM(run.totalMemory()));

System.out.println("free = " + byteToM(run.freeMemory()));

}

public static double byteToM(long num) {

return (double) num / 1024 / 1024;

}

}

运行效果: