Java编写程序，查找一个子串在整串中出现的次数。

可以使用Java的String类的indexOf()方法来查找子串在整串中第一次出现的位置，然后通过while循环不断查找后续出现的位置直到整个字符串中没有该子串，最终得到子串在整串中出现的次数。具体代码如下：

public static int countSubString(String str, String subStr) {
    if (str == null || subStr == null || str.length() < subStr.length()) {
        return 0;
    }
    int count = 0;
    int index = str.indexOf(subStr);
    while (index != -1) {
        count++;
        index = str.indexOf(subStr, index + subStr.length());
    }
    return count;
}

调用该方法时，需要传入需要进行查找的字符串和子串，例如：

String str = "Java is a popular programming language. Java is used for developing a wide range of applications, from mobile devices to enterprise servers.";
String subStr = "Java";
int count = countSubString(str, subStr);
System.out.println("子串\"" + subStr + "\"在字符串\"" + str + "\"中出现的次数为：" + count);

输出结果为：

子串"Java"在字符串"Java is a popular programming language. Java is used for developing a wide range of applications, from mobile devices to enterprise servers."中出现的次数为：2