java 4位byte转为int类型

由于要将一段C语言代码翻译成java

void ReadFile(Point data[][MAX_LENGTH],CString path,int n)
{
CString str="";
Initialize(data);
FILE * fp;
CString filename;
int i,j;
str.Format("%d",n-1);
filename=path;
filename+="/";
for (i=0;i<3-str.GetLength();i++)
filename+="0";
filename+=str;
filename+=".dat";
if ((fp=fopen(filename,"rb"))==NULL)
{
//printf("Can not create the file.\n");
return;
}
int t=-1;
i=0;
j=0;
while (!feof(fp))
{
fread(&t, sizeof(int), 1, fp);
if (t != -1)
{
data[i][j].x=t;
fread(&t, sizeof(int), 1, fp);
data[i][j].y=t;
j++;
}
else
{
fread(&t, sizeof(int), 1, fp);
i++;
j=0;
}
if ((i>=MAX_STROKE)||(j>=MAX_LENGTH))
break;
}
fclose(fp);
}

而最关键的文件操作，一次从文件里面读取四个字节的数据存放到data[i][j].x和data[i][j].y中

由于java只能读取到byte中，写了一个 byte2int的转换h函数

public void ReadFile(Point[][] data, String path, int n) {
Initialize(data);
int i, j;
String str = String.valueOf(n - 1);
StringBuilder filename = new StringBuilder();
filename.append(path).append("/");
for (i = 0; i < 3 - str.length(); i++)
filename.append("0");
filename.append(str).append(".dat");
File file = new File(filename.toString());
FileInputStream fis;
try {
fis = new FileInputStream(file);
i = 0;
j = 0;
byte[] t = new byte[4];
int tint, len;
while ((len = fis.read(t)) <= 0) {
tint = byte2int(t, len);
if (tint != -1) {
data[i][j].x = tint;
len = fis.read(t);
data[i][j].y = byte2int(t, len);
j++;
} else {
fis.read(t);
i++;
j = 0;
}
if (i >= Constants.MAX_STROKE || j >= Constants.MAX_LENGTH)
break;
}
fis.close();
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e1) {
e1.printStackTrace();
}
}

最开始的byte2int转换函数的写法

public static int byte2int(byte[] data,int len) {
int res = 0;
switch(len) {
case 1:{
res = res |data[0];
return res;
}
case 2:{
res = res |data[1];
res = res << 8;
res = res |data[0];
return res;
}
case 3:{
res = res |data[2];
res = res << 8;
res = res |data[1];
res = res << 8;
res = res |data[0];
return res;
}
case 4:{
res = res | data[3];
res = res << 8;
res = res | data[2];
res = res << 8;
res = res | data[1];
res = res << 8;
res = res | data[0];
return res;
}
default:return res;
}
}

然后我发现只有一部分数据正确，而且只要data中存在负数就不正确了，之后在调试之后发现

res = res | data[0] 这一步的时候，实际上是执行的 res = res | (int)data[0] 操作，当data[0]位负数的时候，高位全部补1，导致的结果错误

然后进行改进

public static int byte2int(byte[] data, int n) {
switch (n) {
case 1:
return (int) data[0];
case 2:
return (int) (data[0] & 0xff) | (data[1] << 8 & 0xff00);
case 3:
return (int) (data[0] & 0xff) | (data[1] << 8 & 0xff00) | (data[2] << 16 & 0xff0000);
case 4:
return (int) (data[0] & 0xff) | (data[1] << 8 & 0xff00) | (data[2] << 16 & 0xff0000)
| (data[3] << 24 & 0xff000000);
default:
return 0;
}
}