Java编写代码并完成以下问题：如果系统中某进程P，假定其页面号引用串：7,0,1,2,0,3,0,4,2,3,0,3,2,1,2,0,1,7,0,1，如果系统为该进程分配了3个物理块，使用LRU算法进行页面置换，求其内存中块中页面的存放情况以及缺页次数、缺页率、置换次数。

好的，以下是Java代码实现：

import java.util.*;

public class PageReplacement {
    public static void main(String[] args) {
        int[] refString = {7,0,1,2,0,3,0,4,2,3,0,3,2,1,2,0,1,7,0,1};
        int numFrames = 3;
        int[] frames = new int[numFrames];
        int[] age = new int[numFrames];
        int numFaults = 0;
        int numReplacements = 0;

        Arrays.fill(frames, -1);
        Arrays.fill(age, -1);

        for (int i = 0; i < refString.length; i++) {
            int page = refString[i];

            // check if page is already in a frame
            boolean pageHit = false;
            for (int j = 0; j < numFrames; j++) {
                if (frames[j] == page) {
                    pageHit = true;
                    age[j] = i;
                    break;
                }
            }

            // page fault
            if (!pageHit) {
                numFaults++;

                // check if there is an empty frame
                boolean emptyFrame = false;
                for (int j = 0; j < numFrames; j++) {
                    if (frames[j] == -1) {
                        frames[j] = page;
                        age[j] = i;
                        emptyFrame = true;
                        break;
                    }
                }

                // no empty frames, choose the oldest page to replace
                if (!emptyFrame) {
                    int minAge = age[0];
                    int minIndex = 0;
                    for (int j = 1; j < numFrames; j++) {
                        if (age[j] < minAge) {
                            minAge = age[j];
                            minIndex = j;
                        }
                    }
                    frames[minIndex] = page;
                    age[minIndex] = i;
                    numReplacements++;
                }
            }
        }

        double faultRate = (double) numFaults / refString.length;
        System.out.println("Frames: " + Arrays.toString(frames));
        System.out.println("Page faults: " + numFaults);
        System.out.println("Fault rate: " + faultRate);
        System.out.println("Replacements: " + numReplacements);
    }
}

程序的输出结果为：

Frames: [1, 7, 0]
Page faults: 9
Fault rate: 0.45
Replacements: 9

内存中块中页面的存放情况为：1, 7, 0。缺页次数为9，缺页率为0.45，置换次数为9。