解析webxml遇到一个不会解决的空指针问题

[_凌晨两点半_]

  @Override
    public void characters(char[] ch, int start, int length) throws NullPointerException {
        String contents=new String(ch,start,length).trim();
        if(tag!=null) {
            if (isServlet) {
                if (tag.equals("servlet-name")) {
                    servlet.setName(contents);
                } else if (tag.equals("servlet-class")) {
                    servlet.setClz(contents);
                }

            } else {
                if (tag.equals("servlet-name")) {
                    mapping.setName(contents);
                } else if (tag.equals("url-pattern")) {
                    mapping.addPattern(contents);   //此行报错
                }

            }
        }

public class Mapping {
    private String name;
    private Set<String> patterns=null;

    public Mapping(String name, Set<String> patterns) {
        this.name = name;
        this.patterns = patterns;
    }

    public Mapping() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Set<String> getPatterns() {
        return patterns;
    }

    public void setPatterns(Set<String> patterns) {
        this.patterns = patterns;
    }

    public void addPattern(String pattern){
            this.patterns.add(pattern);   //此行报错

    }
}

Exception in thread "main" java.lang.NullPointerException
at servlet.Mapping.addPattern(Mapping.java:36)
at servlet.WebHandler.characters(Text.java:87)

[dkwuxiang]

private Set<String> patterns=null; 实例化成 new HashSet<String>(); 不就好了，你代码也没截图完整；

// mapping 实例化的问题
else {
                if (tag.equals("servlet-name")) {
                    mapping.setName(contents);
                } else if (tag.equals("url-pattern")) {
                    mapping.addPattern(contents);   //此行报错
                }
 
            }

[_凌晨两点半_]

哈，脑子没转过来，大一刚开始学java，感谢