Java Java类

System 类提供的设施包括标准输入、标准输出和错误输出流；访问外部定义的属性和环境变量；一种加载文件和库的方法；以及一种用于快速复制数组的一部分的实用方法。它扩展类对象。

领域：

public static final InputStream in： “标准”输入流。该流已经打开并准备好提供输入数据。通常，此流对应于键盘输入或主机环境或用户指定的另一个输入源。
public static final PrintStream out： “标准”输出流。此流已经打开并准备好接受输出数据。通常，此流对应于主机环境或用户指定的显示输出或另一个输出目的地。
public static final PrintStream err： “标准”错误输出流。此流已经打开并准备好接受输出数据。
通常，此流对应于主机环境或用户指定的显示输出或另一个输出目的地。按照惯例，此输出流用于显示错误消息或其他应立即引起用户注意的信息，即使主要输出流（变量 out 的值）已重定向到文件或其他目标通常不会持续监控。

方法：

1. static void arraycopy(Object source, int sourceStart, Object Target, int targetStart, int size)：复制一个数组。要复制的数组在 source 中传递，而在 sourceStart 中开始复制的索引将在 sourceStart 中传递。将接收副本的数组在 target 中传递，副本将在 target 内开始的索引在 targetStart 中传递。大小是被复制的元素的数量。

Syntax: public static void arraycopy(Object source, 
int sourceStart, Object Target, int targetStart, int size)
Returns: NA.
Exception: 
IndexOutOfBoundsException - if copying would cause access of data 
outside array bounds.
ArrayStoreException - if an element in the source array could not 
be stored into the target array because of a type mismatch.
NullPointerException - if either source or target is null.

Java

// Java code illustrating arraycopy() method
import java.lang.*;
import java.util.Arrays;
class SystemDemo
{
    public static void main(String args[])
    {
        int[] a = {1, 2, 3, 4, 5};
        int[] b = {6, 7, 8, 9, 10};
         
        System.arraycopy(a, 0, b, 2, 2);
         
        // array b after arraycopy operation
        System.out.println(Arrays.toString(b));
         
    }
}

Java

// Java code illustrating clearProperty(), getProperty()
// and setProperty() methods
import java.lang.*;
import static java.lang.System.clearProperty;
import static java.lang.System.setProperty;
import java.util.Arrays;
class SystemDemo
{
    public static void main(String args[])
    {
        // checking specific property
        System.out.println(System.getProperty("user.home"));
         
        // clearing this property
        clearProperty("user.home");
         
        System.out.println(System.getProperty("user.home"));
         
        // setting specific property
        setProperty("user.country", "US");
         
        // checking property
        System.out.println(System.getProperty("user.country"));
         
        // checking property other than system property
        // illustrating getProperty(String key, String def)
        System.out.println(System.getProperty("user.password",
              "none of your business"));
    }
}

Java

// Java code illustrating console() method
import java.io.Console;
import java.lang.*;
import java.util.Currency;
import java.util.Locale;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
         
        Console c = System.console();
        if(c != null)
        {
           Currency currency = Currency.getInstance(Locale.ITALY);
           c.printf(currency.getSymbol());
            c.flush();
        }
        else
            System.out.println("No console attached");
    }
}

Java

// Java code illustrating currentTimeMillis() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
        System.out.println("difference between the "
                + "current time and midnight,"
                + " January 1, 1970 UTC is: " +
                System.currentTimeMillis());
        System.out.println("current time in "
                + "nano sec: " +
                System.nanoTime());
    }
}

Java

// Java code illustrating exit() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[]) throws NullPointerException
    {
        System.gc();
        System.out.println("Garbage collector executed ");
         
        System.out.println(System.getProperty("os.name"));
         
        System.exit(1);
         
        // this line will not execute as JVM terminated
        System.out.println("JVM terminated");
    }
}

Java

// Java code illustrating gc() method
import java.lang.*;
class SystemDemo
{
    public static void main(String args[])
    {
        Runtime gfg = Runtime.getRuntime();
        long memory1, memory2;
        Integer integer[] = new Integer[1000];
  
        // checking the total memory
        System.out.println("Total memory is: "
                           + gfg.totalMemory());
  
        // checking free memory
        memory1 = gfg.freeMemory();
        System.out.println("Initial free memory: "
                                      + memory1);
  
        // calling the garbage collector on demand
        System.gc();
  
        memory1 = gfg.freeMemory();
  
        System.out.println("Free memory after garbage "
                           + "collection: " + memory1);
  
        // allocating integers
        for (int i = 0; i < 1000; i++)
            integer[i] = new Integer(i);
  
        memory2 = gfg.freeMemory();
        System.out.println("Free memory after allocation: "
                           + memory2);
  
        System.out.println("Memory used by allocation: " +
                                    (memory1 - memory2));
  
        // discard integers
        for (int i = 0; i < 1000; i++)
            integer[i] = null;
  
        System.gc();
  
        memory2 = gfg.freeMemory();
        System.out.println("Free memory after  "
            + "collecting discarded Integers: " + memory2);
    }
}

Java

// Java code illustrating getenv() method
import java.lang.*;
import java.util.Map;
import java.util.Set;
class SystemDemo
{
    public static void main(String args[])
    {
        Map gfg = System.getenv();
        Set keySet = gfg.keySet();
        for(String key : keySet)
        {
            System.out.println("key= " + key);
        }
         
        // checking specific environment variable
        System.out.println(System.getenv("PATH"));
    }
}

Java

// Java code illustrating getProperties() method
import java.lang.*;
import java.util.Properties;
import java.util.Set;
class SystemDemo
{
    public static void main(String args[])
    {
        Properties gfg = System.getProperties();
        Set