You can try something like this (If you want the first 50 words):
String explanation="The image-search feature will start rolling out in the next few days, said Johanna Wright, a Google search director. "Every picture has a story, and we want to help you discover that story she said."
String[] words = explanation.split(" ");
StringBuilder sb = new StringBuilder();
for (int i = 0; i < Math.min(50, words.length); i++)
{
sb.append(words[i] + " ");
}
System.out.println("The first 50 words are: " + sb.toString());
Or something like this if you want the first 50 characters:
String explanation="The image-search feature will start rolling out in the next few days, said Johanna Wright, a Google search director. "Every picture has a story, and we want to help you discover that story she said."
String truncated = explanation.subString(0, Math.min(49, explanation.length()));