A.String x="Hello"; int y=9; x+=y; B.String x="Hello"; int y=9; if(x==y) { } C.String x="Hello"; int y=9; x=x+y; D.String x=null; int y=(x!=null)&&(x.length()>0) ? x.length() : 0;
B 字符串和数字不能用 == 比较